**1. Introduction**

Research on the probiotic effect in preventing or treating diseases has attracted researchers' attention for many decades. The importance of probiotics in such areas can be indicated by close to 300 meta-analyses published from 2000 to 2020 investigating the efficacy of probiotics in preventing and treating diseases. Probiotics are defined as live non-pathogenic microorganisms that confer beneficial health to the host when administered in a sufficient number [1] (1 × 109 colony forming units (CFU) per serving [2]). However, a concern has increased that probiotics may not survive in sufficient numbers when they are added to dairy products or pass through the gastrointestinal tract and may not be helpful as would be expected [3, 4]. Therefore, improving the shelf-life of probiotic strains is important. According to research, microencapsulation of probiotic strain by spray drying through adding additives like tragacanth to skim milk could remarkably enhance the survival of the cells during

drying [5]. Amara and Shibl showed that probiotics are not only helpful in supporting health or managing pathogenic infections, but also effective for the treatment and controlling of diseases [6]. In addition, the utilization of fermented foods which are the usual source of lactic acid bacteria can confer remarkable health benefits, such as decreasing the incidence rate of type 2 diabetes and cardiovascular illnesses [7], and also helpful metabolic effects [8].

Categorizing 294 meta-analysis articles have been done from the year of 2000 until 2020, on the effects of probiotics in the prevention and treatment of diseases, demonstrated that only 21% of these studies reported the ineffectiveness of probiotics in the prevention or treatment of various diseases but 79% showed a positive effect. It is worth saying that no analysis was found to report a negative effect of probiotics. This shows the importance of probiotics on human health that they are not only not detrimental but also may have a beneficial effect on the host. Besides, the statistics of the efficacy percentage of probiotics on diseases indicate that probiotic supplements may be more effective in preventing or treating some diseases (80.95–100%), including; diabetes, infections, irritable bowel syndrome, enterocolitis, and diarrhea. It is worth mentioning that these results have been conducted with more than 14 metaanalysis articles from 2000 to 2020 on each of these diseases (**Figure 1**).

Probiotics have been used to modulate the microbial community in a beneficial way and as a result, immunity improves against many infections that threaten human and animal lives [9]. Probiotics exert their beneficial effects on the host through different mechanisms different mechanisms including straightly eliminating or inhibiting the growth of pathogens by producing antimicrobial substances, destroying toxins, regulating the immune system, reintroducing the microbiota balance, competing with pathogenic microbes for adhesion sites and nutrients, enhancing intestinal barrier function, and immunomodulation [10–13]*.*

Using antibiotics in a large amount unselectively annihilates normal intestine and genital tract flora, and damages the host's mechanisms of immunity [14]. Nowadays, spreading antibiotic resistance among human pathogens is a major public health concern in the world. It can affect people at any stage of life, as well as the healthcare, veterinary, and agriculture industries. As it is clear

### **Figure 1.**

*The effectiveness percent of probiotics on diseases with the highest number of studies (from 2000 to 2020).*

*Probiotic Effects on Disease Prevention and Treatment DOI: http://dx.doi.org/10.5772/intechopen.109717*

antimicrobial resistance is increasing nowadays due to the overuse or misuse of antibiotics against infections; however, probiotics can be used as a great alternative to them [15, 16]. As probiotics help to balance the intestinal microbiota composition, they can protect the host against diseases [17]. However, it should be noted that to produce probiotic supplements for humans or animals, strains containing antibiotic resistance genes must be distinguished from other strains, as there is a possible risk of spreading resistance genes to other pathogenic or non-pathogenic strains [18].

It is the purpose of this chapter to provide a comprehensive review of the research that has been conducted on the importance of probiotics in the prevention/treatment of several common human diseases.
