Population control.

Population control. The only alternative left was to increase the death rate, no one wanted to pick people out of a crowd and line them up for execution, neither did they relish the possible consequences of an enraged public upon discovering that they were being systematically murdered.

A very short but very deadly global war was contemplated.
Something else had to be done that would absolve the decision-makers of guilt, something that could be blamed upon Mother Nature.

The answer, a synthetic biological agent, an agent that does not naturally exist for which no natural immunity could have been acquired.
The United States population was infected. The vaccine was administered by the world health organization.

Whatever causes AIDS was in the vaccine

Over population is the most serious threat to the whole future of our species.
The vaccine was administered by the world health organization.
The cure will be administered to the survivors when it is decided that enough people have died.