The immune system is a collection of molecules, cells, and organs whose complex interactions form a defense network capable of protecting the body from outside invaders. Immune cells, called lymphocytes, are critical to the proper functioning of the immune system. When they encounter foreign cells, some lymphocytes respond by producing antibodies -- large proteins that destroy or otherwise interfere with the vital activities of foreign cells. Other types of lymphocytes, called T cells, either actively kill the foreign cells using powerful chemicals, or secrete chemicals that attract macrophages that eat foreign cells.

A healthy immune system can successfully fight invaders such as the virus that causes mumps. Viruses like the human immunodeficiency virus (HIV), the virus that causes AIDS, however, are not so easily dealt with. This is because the virus attacks the T cells themselves, disrupting the body's natural immune response. To combat the virus's onslaught, the immune system produces a billion new T cells every day. Unfortunately, the viruses also replicate about a billion times each day.

In a person infected with HIV, the body's immune system fights the virus -- day after day, month after month, sometimes year after year. Eventually, though, the immune system exhausts itself. The number of T cells drops dramatically, and without this vital component of the immune system, the body is left susceptible to other diseases.

As has happened with some other diseases caused by microbes, many medical researchers believe that AIDS may one day be cured by a vaccine. Vaccines work by introducing the body to otherwise harmful microorganisms in a safe, controlled manner. Often the microbes in a vaccine are dead or otherwise rendered harmless. Even dead viruses, however, are often enough to elicit the body's immune response -- its ability to recognize and fight off a particular virus should it ever come into contact with the virus again.

In many ways the creation of a vaccine seems like an obvious approach for curing AIDS. It worked surprisingly well in the past. Polio used to kill or maim more than a million people worldwide every year. Today, this disease is but a distant memory. Unfortunately, the vaccine approach worked as well as it did with diseases like because the human immune system already had some innate ability to fight the viruses that caused them. Generation after generation of human populations had evolved with these viruses and, thus, had become better at fighting them and the diseases they cause. Indeed, humans have at least some natural ability to fight most viruses. That, unfortunately, is not the case with HIV, perhaps because human beings have not been exposed to the virus for long enough.

Recent findings suggest that HIV has been in the human population for only a short time. The virus, according to at least one study, probably jumped from chimpanzees to humans very recently -- maybe only a few decades ago. If this is the case, researchers surmise that our species would not have had time to evolve resistance to the virus's effects, as it seems chimpanzees and a few other species with viruses similar to HIV have. Despite the setback in what some thought would be a simple solution to a global epidemic, many researchers remain committed to the idea of an AIDS vaccine. Toward this end, some are studying a small group of people -- perhaps 5 percent of the HIV-positive population -- who have the virus but don't show any symptoms and whose infection remains at very low levels. Many experts believe these people's immune systems have learned to fight the virus differently than other people's -- in a way that the body could "learn" through a vaccine.