The AIDS virus is difficult to fight in part because it attacks immune system cells and in part because it is constantly mutating, making it a constantly moving target. Researchers have been looking for parts of the virus that do not mutate so they can design vaccines that will protect against these constantly changing versions. Even a vaccine that is not 100 percent effective or is not given to 100 percent of at-risk populations could have significant benefits. According to the International AIDS Vaccine Initiative (IAVI), a vaccine that is 50 percent effective and given to just 30 percent of the population could cut the number of new HIV infections in developing countries by 24 percent over 15 years.
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