AIDS has been able to infect and kill so many people because of the way it works. Let's look at some of the features that make this disease so unusual.
HIV spreads through intimate contact with an infected person. Forms of intimate contact that can transmit AIDS include sexual activity and any sort of situation that allows blood from one person to enter another. When compared with the many viruses that spread through the air, it would seem that the intimacy involved in the transmission of AIDS would be a limiting factor.
However, a person can be contagious for a decade or more before any visible signs of disease become apparent. And in that decade, an HIV carrier can potentially infect dozens of people, each of whom can infect dozens more, and so on.
HIV invades the cells of our immune system and reprograms them to become HIV-producing factories. Slowly, the number of immune cells in the body dwindles and AIDS develops. Once AIDS manifests, a person is susceptible to many different infections because HIV has weakened the immune system to the point where it can no longer fight back effectively.
In fact, HIV not only invades and weakens the immune system — the very system that would normally protect the body from a virus — it destroys it. HIV has also shown the ability to mutate, which makes treating the virus very difficult.