Infectious diseases are diseases caused by different species that are usually too small to be seen by the naked eye. These organisms are bacteria, viruses, fungi, and parasites and tend to spread from one person to another in direct or indirect form. One can also contract the disease by contacting an infected animal that carries a pathogenic bacterium that causes the disease in humans. Infectious diseases are the leading cause of death globally and are most predominant in countries with low income and particularly the young population. Reduced respiratory diseases and diarrhoea are the two communicable diseases WHO ranked in the top ten globally, leading to mortality globally in 2019. Many microorganisms have the proven capacity to cause either or both of these defects. Over the past few years, there has been a sharp decline in mortality, mainly from diseases of infectious nature such as HIV/AIDS, Dengue, Malaria and Nipah. These diseases are no longer among the top ten most common diseases leading to death across the globe. On the other hand, these illnesses remain some of the leading causes of death in Countries with low incomes. Another communicable disease that ranks high on the list of diseases that causes mortality in countries with low per capita income is malaria. Only one infectious agent causes each of these three diseases. Infectious agents can be microorganisms, but most microorganisms do not make people sick. The microbiome can be described as bacteria living in the human body and performing several beneficial and inevitable tasks. The number of infections resulting in man's illness is principally due to bacteria or viruses; however, malaria is caused by a protozoan, which is quite peculiar. This chapter discusses the mechanism of several types of infectious viral and protozoan diseases.