INFECTIONS

Infections are caused by microorganisms that attach themselves to body cells and replicate. These microorganisms include bacteria, viruses, fungi, protozoa (single-celled organisms), lice and helminths (parasites and worms).
To facilitate an infection, three things must be present:
A microorganism
A vulnerable host
A means of spreading (this is called transmission)
All infections have the potential to cause a person to become sick, though most are not harmful as the body and immune system provide various forms of protection.