**2.1 Plants and our life (plants as pillar of life)**

Imagine you woke up one day to the realization that every plant has died. Now, create a mental picture of this incident. What will the world look like—a world without plants? There will be no food. There will be no shelter. There will be no clean air. There will be a bad water cycle. And soon there will be no animals as well as other

#### *Plant Medicine and Infectious Disease DOI: http://dx.doi.org/10.5772/intechopen.103701*

organisms. Plants support essentially every life. Together with animals, other organisms, and the abiotic aspect of the ecosystem almost every life is impacted.

Plants are all the organisms belonging to the kingdom Plantae. They are eukaryotes capable of using water, carbon dioxide, and the energy from the sun to produce chemical energy which is stored and, in the process, releases oxygen into the atmosphere which animals and other aerobic organisms depend on for survival. The chemical energy, an end product of photosynthesis, is taken up by animals in the form of food and released through cellular respiration to support life.

Let us step back and analyze well the importance of plants to life. Within the ecology, there exists a sequence of matter and energy transfer from organism to organism. Every organism requires matter and energy for carrying out life processes. These basic requirements have to be transferred from one organism to another hinging on some extent of conservation. This means of matter and energy transfer between organisms is termed the food chain. The food chain is nothing more than feeding relationships between species in a biotic community. These feeding relationships start at the base with a producer, then the primary consumer, the secondary consumer, tertiary consumer, and so on. This trend implies that the producer has to find a way to offer the 'first' matter and energy needed to start the transfer and that is what they do well. The producers make their food which is in turn taken up by the primary consumer. The primary consumers are also in turn taken up by the secondary consumer which in turn are also taken up by the tertiary consumers and so on. The primary producers, the most important aspect of the food chain, are most often plants (and other autotrophs organisms—algae, cyanobacteria, and chemoautotrophs). Every organism involves in the food chain and food webs (several interrelated food chains) to retrieved energy to carry out its vital process which would have been a fantasy without the producers like plants.

What more? Plants are one of the agents in the oxygen cycle. Plants manufacture the majority of the air we breathe through photosynthesis (the same process through which they give us food.). The human body has water (H2O) being the majority component that is formed from hydrogen and oxygen (the oxygen we get from plants is part of this oxygen). Aside from plants giving as essential oxygen needed for life, they purified the atmosphere by removing chemicals dangerous to life. The carbon dioxide we breathe out is exchanged for fresh oxygen. At no time in history has there been a rise in the Industrial Revolution than our time. The high demand of the Industrial Revolution presents its mayhem to man and the environment as the result of burning fossil fuel releasing dangerous chemicals into the environment. Despite the frequent burning of fossil fuel and the destruction of vegetation both on land and sea, we still enjoy the relatively stable level of atmospheric oxygen. What can we attribute this to other than the plants that we are still sharing the environment with?

Moreover, plants serve as the habitat for thousands of organisms. In, on, and under plants are thousands of animals that all contribute one way or the other to the existence of the ecosystem. Walk through any forest and you will be amazed at the diversity of life that the forest harbors. Tree kangaroo, giant panda, monkeys, rabbits, foxes, raccoons, squirrels, chipmunks, badgers, moose, bear, bobcats, deer, antelope, lynx, jaguars, elephants are to mentioned but few of wildlife that would have been homeless without the forest. Within the human settlements, plants provide shades moderating temperature, protecting the lands and settlements from wind and water destructions. Again, ornamental plants are employed in almost all human settlements for beautification purposes. Even after cutting plants and destroying their lives they

still give us logs from which many products including paper, furniture, houses, and more goodies that make our lives comfortable are retrieved [12].

#### **2.2 Plants as medicine**

Aside from all the enormous benefits of plants to mankind, plants remain the center of what heals us. Readers may be surprised to know many conventional drugs are from plants (e.g. **Table 1**). Aspirin, a nonsteroidal anti-inflammatory drug (NSAID) used in inflammation and as an analgesic was from Willow; Digoxin, a cardiac stimulant used in congestive heart failure and cardiac arrhythmia were from Digitalis; quinine from Cinchona, morphine from Papaver, and artemisinin from Artemisia. Across the world especially in Third World countries plant medicine has been integrated into the Primary Health care delivery system. People in Asia, Africa, and Latin America depend to a greater proportion on phytomedicine in the treatment of illness partly caused by the great vast of plant species in their part of the world. No apology should be made for placing plants at the center of man's source of healing.

The use of plants for the treatments of diseases predates the records of human history. History has it that plants were the source of remedies of Pythagoras, Galen, and Hippocrates. The earliest human ancestors discovered the healing potency of plants through factors such as instinct, taste, observation, and experience. By relying on trial-and-error, medicinal plants were differentiated from poisonous plants. However, in many cases, the when and how these medicines were first used was lost in pre-history. Pieces of knowledge of medicinal plants, harmful plants, and their mode of usage were passed from generation to generation through oral transmission. Later, writing replaced the oral transmission mode of conveying knowledge from generation to generation (e.g., the Egyptians Ebers Papyrus). These written pieces of knowledge became the great wealth of knowledge upon which further development and discoveries were made.

Though the ancients men depended on trial and error with the first patients as an experiment in screening their plant medicinal preparations, plants are now screened depending on the modern scientific methods of investigation which comprises of a team of botanists (a scientist specialized in the study of plants), phytochemists (a scientist specialized in the branch of organic chemistry dealing with the chemistry of plants), pharmacologists (a scientist trained in the sciences of drugs) and clinicians (a practitioner of medicine whose work focus more on clinical work other than laboratory experiments).

The potency of plants as medicines depends on their phytochemical constituents. Phytochemist focuses on these compounds which have been divided into Primary Metabolites and Secondary Metabolites. Primary metabolites which include amino acids, lipids, carbohydrates, proteins, and vitamins play physiological roles such as growth, development respiration, storage, and reproduction. Secondary metabolites are organic compounds produced by plants through metabolic pathways derived from the primary metabolite's pathway. These metabolites (secondary metabolites) are not directly involved in the basic life processes of the organism but they are essential in other activities. The identification of secondary metabolites opened the gateway for the use of plants as medicine. These compounds are shown to have biological activities. The secondary plant metabolites are grouped into Alkaloids, Saponins, \*Lipids, Phenolics, Terpenes, and \*Carbohydrates.

Alkaloids are nitrogenous compounds (at least a nitrogen atom that usually forms part of a heterocyclic ring structure with marked physiological actions in



#### **Table 1.**

*Medicinal plants as a source of modern medicine [13].*

man and animals). Alkaloids are widely distributed in the Kingdom Plantae especially in angiosperms (higher plants). Alkaloids are rarely found in lower plants. Commonly known plant families in Angiosperm with alkaloids include Papaveraceae, Apocynaceae, Rubiaceae, Berberidaceae, Solanaceae, Convolvulaceae, Ranunculaceae, etc. Example of alkaloids includes Caffeine (central nervous system agent with effects on respiratory and cardiovascular systems.), Vinblastine (an antineoplastic drug from *Catharanthus Roseus*), Nicotine (ganglionic cholinergic receptor agonist), etc.

Saponins are secondary metabolites that contain a high percentage of glycosides (saponin) which can produce stable frothing aqueous solution. Saponins are harmful by their ability to cause hemolysis when injected into the blood but are harmless when taken by mouth. Saponins are proven to possess pharmacological activities including analgesic, antineoplastic, etc.

Lipids are a heterogeneous group of compounds with long-chain fatty acids and glycerol and higher monohydric alcohols. Lipids include fixed oils, fats, waxes, lecithins, phosphatides, etc.; which are relatively insoluble in water but dissolve in organic solvents. Lipids are primary plants metabolites but have been known to possess pharmacological activities.

Phenols probably constitute the largest group of plant secondary metabolites. The phytochemistry of phenols reveals the possession of at least one or more groups of phenol. Phenolic classes of pharmaceutical interest include simple phenolic compounds, tannins, anthraquinones, and their glycosides, coumarins and their glycosides, naphthoquinones, flavone, and related flavonoid glycosides, anthocyanidins and anthocyanins, and lignans and lignin. Phenols have pharmacological properties including antioxidants, anti-inflammatory, etc.

Terpenes are secondary plants metabolites derived from 5-carbon isoprene units. The number of the isoprene units is the basis for terpenes classification. Prefixes such as Hemi, Mono, Sesqui, etc., are used before terpenes to represent the number of isoprene units which in this case are 1, 2, and 3 isoprene units respectively. Hemiterpenes represent a terpene with 1 isoprene unit and so on.

Carbohydrates just like lipids are primary metabolites but form part of a majority of secondary metabolites through glycoxidation linkage. Carbohydrates such as mucilage are used as demulcents employed in ulcer management and inflammatory digestive disorders [14].

#### **2.3 Plants medicine and infectious diseases**

At the dawn of civilization, dating back to hundreds of years, snake venom was drunk intending to confer immunity to snake bites. At a similar time, it was found that smearing cowpox on torn skin induces the body to fight against smallpox. Forwarding to 1796, Edward Jenner who is popularly known as the founder of vaccinology in the West was successful in his attempt in the discovery of a solution that will

#### *Plant Medicine and Infectious Disease DOI: http://dx.doi.org/10.5772/intechopen.103701*

later impact civilization till this day by inoculating a 13-year-old-boy with vaccinia virus (cowpox) and demonstrated immunity to smallpox leading to the discovery of the smallpox vaccine three years later. After this concept underwent medical and technological changes the years ahead, the concept of vaccine and vaccination has been employed across almost every corner of the world and enormous benefits reaped as well.

Despite medical and technological advancement in vaccinology leading to the availability and use of effective vaccines and antibiotics, the concept of infectious diseases still presents a great canker to humanity. To this day that vaccine can be produced in the shortest possible time made possible by the advancement of scientific knowledge leading to research costing millions of dollars, infectious diseases remain an essential problem to the world. Covid'19 outbreak since 2019 has claimed millions of lives across the world daring the elderly and the weak with low immunity as a result of old age, chronic diseases, immunosuppressive drugs, malnutrition, etc., to survive its blow. According to Robbins and Cotran's book, "Pathologic Basis of Disease," Eight Edition, more than 10 million people die each year of infectious diseases [15].

Infectious disease is an impairment of health or a condition of abnormal functioning caused by microorganisms mainly through a specific kind of contact. They can be caught through contact with infected people, infected environments, infected animals, or insect bites. An example includes acquired immunodeficiency syndrome (AIDS), Viral hepatitis, Syphilis, Lyme Disease, Common Cold, Giardiasis, Malaria, Influenza, Measles, Pneumonia, Salmonella infections, Tuberculosis, Whooping cough (pertussis), Rubella, Shingles, Onchocerciasis, Candidiasis, Aspergillosis, Clostridial Infections, etc. Agents responsible for infectious diseases include viruses, bacterial, prions, fungi, helminths, protozoa, etc.

The incessant evidence of the antimicrobial resistance and side effects of synthetic drugs which have been the mainstay of remedying infectious diseases should straighten the line that was once burnt and bring the attention of the global population to the importance of medicinal plants and traditional therapeutic system in remedying infectious diseases.

An attempt to use natural products (plant medicines) to remedy infectious diseases swings on two pivots. The first is to consider factors related to the patients. These factors which must be considered relating to the patients include the patient's previous health histories, susceptibility to infections, chronic diseases, ability to tolerate drug by mouth, age, sex (if female, whether with a child or pregnant). The second factor to consider is that relating to the type of infections and the causal organism.

What follows describes the infectious agents responsible for infectious diseases and further recommends plants medicines for their remedy or which has shown efficacy against such specific infectious agents.

#### *2.3.1 Viruses*

Viruses are submicroscopic obligate intracellular infectious agents, which by definition consist of genetic code, either DNA or RNA, enclosed by a protein coat that is sometimes encased in a lipid membrane. Viruses cannot replicate on their own. They, therefore, depend on the host cell's complex metabolic and biosynthetic machinery for their replication. Viruses as infectious agents can infect animals, plants, and microorganisms such as bacteria and archaea. Viruses exist outside their host as virions (also used to describe a fully assembled infectious virus), an inert independent viral particle; nucleic acid, and capsid (and a lipid envelop in some cases). Viruses such as

polio and tobacco mosaic virus can even be crystallized. With an infection, the main function of the virion is to deliver the genetic component (DNA or RNA genome) into the host cell so that the genome can be transcribed and translated by the host cell. Viruses are known to be the most numerous types of biological entities. There are millions of species of virus existing with only a few given a detailed description.

Viruses are grouped based on their nucleic acid genome (DNA or RNA and not both), the shape of their capsid (helical or icosahedral), the presence or absence of a lipid envelop, their mode of replication, the preferred cell type for replication (atropism), or the type of pathology.

Viruses are responsible for a great share of human infections including AIDS, Corona Virus Infection, Chicken Pox, Common Cold, Dengue fever, Lassa fever, Herpes zoster, Herpes simplex, measles, SARS, Rabies, Smallpox, Viral meningitis, Viral pneumonia, Viral gastroenteritis, Yellow fever, Cytomegalovirus infection, Ebola hemorrhagic fever, etc. Some viruses are responsible for transient illnesses such as colds and influenza while others are hard to be eradicated from the body. Many viruses have a propensity to cause latent infections by expressing their genome into viral protein. The majority of these viruses belong to the *Herpesviridae* family: Epstein-Barr virus, Varicella-Zoster Virus, etc. Hepatitis B virus is known in several cases to persist in the host cell for a longer duration of time causing latent illnesses which can be reactivated later. The human papillomavirus induced benign warts and cervical carcinoma. The same clinical manifestation can be depicted by different species of virus and the same virus can depict different clinical manifestations depending on factors such as host immune status and age. Viruses can spread through touch, saliva, and through the air, sexual contact, insects (known as vectors), and through sharing of contaminated items [16].

### *2.3.2 Plant medicines for viral infectious*

Viral infection presents a lot of challenges in the health industry. When a living cell is invaded by viruses, they hijack the cell's internal machinery to produce more of their kind in the process destroying a lot of host cells. Viral infections remain an area of medicine for which specific treatments are lacking. In immunocompetent individuals, some viral infections resolve spontaneously on their own. For most viral infections such as those associated with the common cold, treatment involves symptoms relief and not targeting the viral agents per se. Antiviral agents are agents that target the viral agent preventing their replication in the host cell. These agents include plants. *Alchornea cordifolia* is used in Ghana as an antiviral agent [17]. Clausena *anisata* contains carbazole alkaloids that inhibit Epstein-Barr virus early antigen in Ragi cells [18]. *Bridelia ferruginea* contains the flavonoids quercetin, quercitrin, and rutin that have demonstrated antiviral effects against coxsackie, Herpes simplex, measles, parainfluenza, and polioviruses [19]. *Scoporia dulcis* contain scopadulcic acid C which potentiates antiviral effect [20]. Extract of *Momordica charantia* has demonstrated antiviral activity against *Herpes simplex* virus type 1 and the proteins alpha and betamomorcharin have been reported to inhibit HIV *in vitro*. Other antiviral agents include *Argemone Mexicana* (leave and stem), *Aloe vera, Allium sativum, Acacia nilotica, etc.*

#### *2.3.3 Bacteria*

Bacteria are free-living organisms that are found almost everywhere on Earth. Bacteria are prokaryotes, meaning they have a cell wall mostly bound by peptidoglycan, a polymer of glycan and peptides, but lack a distinct nucleus and other organelles

#### *Plant Medicine and Infectious Disease DOI: http://dx.doi.org/10.5772/intechopen.103701*

due to the absence of internal membranes which distinguish them from eukaryotes. Bacteria are classified based on shape, Gram staining, and need for oxygen. There are three basic shapes. The spherical bacteria are spherical and known as the cocci, the rod-shaped bacteria are known as bacilli (others curved known as the vibrio) and the spiral-shaped bacteria are known as the spirilla. Under gram staining, bacteria can be either classified as gram-positive or gram-negative based on the bacterial cell wall's ability to retain the crystal violet dye during solvent treatment. The gram-positive bacteria retain the dye because of their single thick cell wall and appear purple-brown under the microscope and the gram-negative bacteria do not take up the dye because they have a thin cell wall squash in between two phospholipid bilayer membranes and appear red under the microscope. Bacteria that require oxygen for survival are aerobic bacteria and those that can survive where there is no oxygen are the anaerobic bacteria. Unlike viruses, some bacteria can survive and replicate outside their host. Not all bacteria are harmful. Some bacteria are used in industrial and medicinal processes. Bacteria help in the fixation of nitrogen into the soil from the atmosphere and are vital to the planet's ecosystem. The human body serves as a home for many species of bacteria in a symbiotic relationship. However, several species of bacteria have broken this symbiotic relationship and are pathogenically responsible for many infectious diseases including anthrax, cholera, bacterial meningitis, brucellosis, diphtheria, gonorrhea, Lyme disease, leptospirosis, Pneumococcal pneumonia, syphilis, tetanus, trachoma, tuberculosis, typhoid fever, Q fever, Pertussis, shigellosis, Rocky Mountain Spotted Fever, salmonellosis, tularemia, nocardiosis, campylobacteriosis, etc. [21–23].

### *2.3.4 Plant medicines for bacterial infections*

History has it that antibiotics were limited to substances produced by microorganisms. In 1877 Louis Pasteur discovered that injecting Bacillus anthracis in animals protected them from developing anthrax. Years after Pasteur's discovery, Fleming observed that a colony of Penicillium notatum contaminant on Petri dish inhibited bacteria which lead to the discovery of penicillin. The trend continues leading to the discovery of other compounds. Long before this array of discoveries, nature has already instituted compounds in plants with antibacterial properties waiting to be discovered. Plants' source of antibiotics includes lichen, a thallophytic plant of the division Lichenes that occur as crusty patches or bushy growths on tree trunks, walls, roots, or bare grounds. Lichenes contain usnic acid or vulpinic acid that is known to possessed bacteriostatic properties. Plants belonging to the Order Coniferae have also demonstrated antibacterial activities; essential oils in Juniperus and Pinus spp. possess antibacterial activity. A sulfur-containing amino acid in garlic (alliin) has antibacterial activities. Aloe vera gel and ginger have antibacterial properties. Sesquiterpene ketones in dicotyledons such as hops (humulene and lupulene) and myrrh (furanodiene-6-one and methoxyfuranoguaia-9-ene-8-one; protoanemonine, in *Anemone pulsatilla* and many Ranunculaceae; sulfur-containing compounds in the Cruciferae; plumbagin in Drosera have all demonstrated antibacterial activities [24].

#### *2.3.5 Fungi*

Fungi are all the known species of organism belonging to the kingdom Fungi. Fungi include yeast, molds, smuts, mushrooms, and toadstools. Fungi are distinct from green plants. Fungi are eukaryotic organisms and possess thick chitin in their cell walls which separates them from other eukaryotic organisms and

ergosterol-containing cell membranes. Fungi can be found almost everywhere on earth. They are with both harmful and beneficial effects. Their characteristics are intermediate between algae and protozoa. They share with plants some characteristics which include possession of cell walls, liquid-filled intracellular vacuoles, microscopically visible streaming of cytoplasm, and they lack motility. Just like bacteria, fungi aid the return of scarce materials to the soil by decomposing animals and plants. Fungi exhibit mycorrhizae association with plants (a symbiotic relationship between the mycelium of a fungus and the roots of a vascular plant) in which they provide plants with some nutrients and water for growth and taking nutrients as well from the plant. Medicines such as penicillin (an antibiotic) were produced from the fungus *Penicillium notatum*. Fungi are also used in the agricultural industry for controlling insect pests on crops. Most species of fungi live in the soil where they can obtain their nutrients, and on living organisms including plants and animals. Fungi again are saprophytic organisms and feed on dead organic matter. Fungi just like animals are heterotrophic organisms (they lack chlorophyll) and depend on the absorption of dissolved organic molecules typically by secreting digestive enzymes into their setting. Despite the enormously beneficial effects of fungi, some fungi are responsible for a wide range of infectious diseases infecting both animals and plants. Fungi are responsible for infectious diseases classified into superficial, cutaneous, subcutaneous, systemic, and opportunistic infections. Superficial diseases involve the skin, hair, and nails. Some fungi invade the subcutaneous while others invade deep tissues destroying internal organs in the immunocompromised host. Opportunistic fungi are generally harmless in their normal environment but become harmful in an immunocompromised host. Infectious diseases caused by fungi include tinea, candidiasis, coccidioidomycosis, aspergillosis, blastomycosis, histoplasmosis, etc. [25].
