For decades, they lived a mystery: Why were they able to survive with the AIDS virus, free of symptoms and the need for potent drugs, while so many others with the same germ turned deathly ill?

Their innate ability to keep HIV infections in check intrigued researchers, who suspected these people, known as “controllers,’’ might carry clues to designing effective vaccines after nearly 30 years of frustration.

Now, an international team of researchers, led by specialists in Boston, has cracked these HIV survivors’ genetic code, sifting through almost 1.4 million pieces of DNA to discover five amino acids that separate the small cadre of controllers from the vast majority who must take medication or face death.