Primary prevention means
preventing illness before it occurs. Immunizations (vaccines) are one kind of
primary prevention. Medicines that kill or control the organisms that cause
infections are another type of primary prevention.
Secondary prevention means preventing a disease that a person
has already had from coming back. This is usually done with medicines that slow
or prevent the growth of the organisms that cause infections.
Generally, infection with HIV doesn't make people sick, except for
the flu-like illness that may develop shortly after they become infected. Most
people who are infected with HIV get sick because their
immune systems become weak and cannot fight off other
infections. So preventing opportunistic infections is an important part of
treatment for HIV.