define infectious diseases
Infectious diseases. WHO/Chris Black. Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another.Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans.nfectious diseases. WHO/Chris Black. Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another.