Here are some ways for diseases to defeat the immune system:
Some diseases reproduce so rapidly
that the immune system
doesn't have time to respond.
ATTACK THE IMMUNE SYSTEM.
HIDE FROM IT.
Some diseases hide in places where the immune cells have trouble going.
Tuberculosis, for example, can hide out for years in the lungs.
INFECT PEOPLE WITH WEAK IMMUNE SYSTEMS.
Children,the elderly, and people with HIV are particularly at risk.
You especially fear people who have been vaccinated against you. Their
immune systems are already primed and ready to destroy you. The only
defense is to be a type of disease with many different strains. The
host will have to be vaccinated against each strain, which is difficult
(and sometimes impossible) to do.