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.
Infectious diseases also include emerging infectious diseases; diseases that have newly appeared (e.g., Middle East Respiratory Syndrome) or have existed but are rapidly increasing in incidence or geographic range (e.g., drug-resistant tuberculosis (XDR TB) and COVID-19
Most often, illness is directly caused by the infectious agent or its toxic product, which arises through transmission from an infected person, an infected animal, or a contaminated inanimate object to a susceptible host.
Infectious disease control and prevention relies on a thorough understanding of the factors determining transmission
