**3. Epidemiology**

1884 and 1884 respectively identified two of the most common bacterial causes of pneumonia, *Streptococcus pneumoniae* and *Klebsiella pneumonia* [3]. By the 1930s, treatment for pneumonia

Pneumonia remains the leading cause of childhood mortality under the age of 5 and the most common reason for adult hospitalisation in low and middle income countries, despite advances in preventative and management strategies [4]. Pneumonia usually causes symptoms for 3–4 weeks, and daily activities may be impaired for a further 3 weeks on average. Community-acquired pneumonia (CAP) refers to pneumonia acquired outside of hospitals or extended-care facilities. Nosocomial pneumonia and hospital-acquired pneumonia describe infections acquired in the hospital setting. These are usually defined as pneumonia that occurs 48 h or more after hospital admission, and which was not incubating at the time of admission. Community-acquired pneumonia continues to be a significant health issue [5]. Annually in the United States there are around 4 million cases of which 20% of cases may require hospitalization. As a result there are more than 65 million days of reduced activity overall. Mortality rates can range from 1 to 30% making it the sixth leading cause of death [6]. In developing countries pneumonia is either the first or second leading cause of death. In Europe, around 14.4 per 10,000 children aged over 5 years and 33.8 per 10,000 under 5 years are diagnosed with CAP. CAP is more common in the developing world, estimated at 0.28 episodes per

Pneumonia is an inflammatory process in lung parenchyma most commonly caused by bacteria and viruses. Less common etiologies include mycoplasma, fungi and parasites. Organisms spread to the lungs through aerosolization, aspiration, or hematogenous spread due to inhalation of droplet or by aspiration of fluids in the oropharynx [8] Pneumonia results if host defense mechanisms are unable to keep the respiratory network infection free. The pathophysiology varies depending on etiology. In the case of bacterial pneumonia there is an intra-

In the case of viral pneumonia there is an inflammatory interstitial inflammation with infiltrate in the alveolar, causing damage to ciliated epithelium surfaces. The lungs become congested, hemorrhagic and Intracellular viral inclusions may form. Local host defenses, such as mucociliary clearance, or secretion of specific secretory IgA antibodies can remove some of the virus particles. However, if mucociliary clearance is impaired or secretory anti-influenza IgA antibodies are absent, infection continues to spread. Respiratory epithelial cells are invaded, and viral replication occurs. Newer viruses then infect larger numbers of epithelial cells, shut off the synthesis of critical proteins, and ultimately lead to host cell death [10].

There can be numerous types of immune response depending on how cytokine is produced. For example, cell-mediated immunity is initiated in type 1, while type 2 cytokines are responsible for allergic responses. Children infected with respiratory syncytial virus (RSV) with more serious acute bronchiolitis often have impaired type 1 immunity or augmented type 2 immunity [11].

had been developed with the introduction of penicillin playing a key role.

child per year and accounting for 95% of all cases [7].

alveolar suppurative exudate with consolidation [9].

**2. Pathophysiology**

192 Contemporary Topics of Pneumonia

Numerous studies by the WHO have estimated there are over 450 million cases of pneumonia globally with approximately 3 million deaths particularly prone are the elderly and children [14]. The annual rate of CAP increases from 6/1000 in the 18–39 age group to 34/1000 in 75 years and over age group. Incident rates tend to be higher in colder climates of the North and hospitalization is required in 20–40% cases. In severe cases mortality can vary from 5 to 10% of cases [15] .

Viral pneumonias are common in the Mideast. In an Iranian study viruses causing pneumonia were Influenza A (7.4%), influenza B (3.5%), RSV (12.9%), and adenovirus (5.9%). Parainfluenza-1,2 and 3 were 6.4, 6.4 and 15.8% respectively [16]. More recently, avian influenza has become endemic in some parts of the Middle East, especially Egypt and Turkey [17].

WHO data published in May 2014 Influenza and Pneumonia Deaths in Saudi Arabia reached 5689 or 7.08% of total deaths. The age adjusted death rate is 44.89 per 100,000 of population [18]. Middle East respiratory syndrome is caused by a novel coronavirus (MERS-CoV) first isolated in the Kingdom of Saudi Arabia in 2012 from the respiratory tract secretions of a Saudi businessman who died from viral pneumonia [19]. Subsequently, cases were identified in patients living outside the Arabian Peninsula and the Middle East, who were infected either during a stay in the Middle East or by close contact with an individual from an endemic country. Most affected patients were previously healthy men with a median age of 50 years [20]. In 2016, the World Health Organization (WHO) published a report on 1698 laboratoryconfirmed cases of MERS-CoV infection. The mortality rate was 36%. All cases were directly or indirectly linked through residence or travel to Saudi Arabia, the UAE, Jordan, Qatar, Oman, Lebanon, Kuwait, Yemen, Egypt, and Iran. There were also reports of sporadic reports in other countries including the United Kingdom, France, Malaysia, Tunisia, Italy, Austria, Greece, Turkey, the United States of America, Germany, Philippines, and Thailand [21]. The largest outbreak of the virus outside its endemic region was recorded in 2015, in South Korea. One-hundred and eighty-six additional cases were confirmed, including the first in China, with a total of 36 deaths. MERS-CoV is a zoonotic virus that can lead to secondary human infections. Dromedary camels are considered as the intermediate host, with closely related virus sequences in bats. Human-to-human transmission has been noted in households and health care setting. But community-wide transmission has not been observed [22].

**4.1. Organisms that cause atypical pneumonias include**

enza viruses 1, 2 and 3, rhinoviruses, and coronaviruses [34].

measles [33].

**5. Viral pneumonia**

pneumonia [35].

They include:

risk of severe RSV infection [36].

*Mycoplasman pneumoniae*, the most common atypical pneumonia organism spreads when someone carrying the infection comes in close contact with others. The condition, also known as "walking pneumonia," is generally mild and seen in the outpatient setting. It appears to occur mostly in school-aged children and young adults. Less common is *Chlamydia pneumoniae* which causes 10% of all CAP cases and is usually mild but usually more severe in the elderly [31]. *Legionella pneumophila* causes Legionnaires' disease commonly found in hotels, cruise ships, hospitals and commercial buildings, where people come into contact with contaminated droplets from cooling towers and evaporative condensers. Other reports of infection have been noted near whirlpools and saunas [32]. It is believed the organism causes up to 4% of all pneumonia cases. Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS) and

Pneumonia of Viral Etiologies

195

http://dx.doi.org/10.5772/intechopen.71608

The advent of molecular diagnostics has greatly improved the identification of viruses in patients with CAP. Over the last decade, several studies have used PCR to establish the importance of viruses in the etiology of CAP. Globally, it is estimated that 200 million cases of viral pneumonia occur annually. Most commonly are influenza viruses (A and B), parainflu-

Viral pneumonia prevails mostly in young children and older adults. Etiologies include influenza, adenovirus, parainfluenza, H1N1 and respiratory syncytial virus (RSV). Influenza A and B occurs in the winter and spring. Symptoms include, headache, fever, and muscle aches. Respiratory syncytial virus (RSV) is most common in the spring and infects young children. Adenovirus and parainfluenza viral pneumonias exhibit cold symptoms (runny nose and conjunctivitis). Post-influenza pneumonia is often accompanied by secondary bacterial infection due to *Staphylococcus pneumoniae* and *Staphylococcus aureus.* Pneumonia in immunocompromised patients is attributed to measles, HSV, CMV, HHV-6 and Influenza viruses. There is also an increased risk of secondary bacterial lower respiratory tract infection (LRTI). The known complication following influenza infection is *Staphylococcus aureus*

Respiratory syncytial virus (RSV) has been identified as an important cause of pneumonia in adults, especially in the elderly. The rate of RSV, overall is between 2 and 5% throughout the year and between 5 and 14% during winter. Adults with severe immunodeficiency are at particular

Viral pneumonia infections include both DNA and RNA viruses. Some are well-known lung pathogens that produce common clinical and radiologic manifestations. Others are rarely involved as lung pathogens. Many viruses can cause pneumonia, either directly or indirectly.
