Infectious Deisease Deifinition. Viruses that invade and multiply inside healthy cells. An infection is the invasion of an organism's body tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce.
Viruses are sometimes referred to as microbes because, like microbes, they are microscopic and can cause disease. Put simply, an infectious disease happens when bad germs get into the body in some way, spread, and make you sick by affecting the way your body normally works. Each disease is associated with a set of signs and symptoms.