Agents that Cause Infectious Diseases

Infectious diseases can be caused by several different classes of pathogenic organisms (commonly called germs). These are viruses, bacteria, protozoa, and fungi. Almost all of these organisms are microscopic in size and are often referred to as microbes or microorganisms. Although microbes can be agents of infection, most microbes do not infect humans or cause disease in humans.
The majority of agents that cause disease in humans are viruses or bacteria, although the parasite that causes malaria is a notable example of a protozoan.
Examples of diseases caused by viruses are HIV/AIDS, influenza, SARS, smallpox, diarrheal diseases, hepatitis, Ebola, and West Nile. Examples of diseases caused by bacteria are anthrax, tuberculosis, salmonella, respiratory and diarrheal diseases.