One virus, causing agonizing pain, kills a person in less than 2 weeks. The other, stealthily weakening its host's immune defenses, can take years to bring about death. Before a worldwide vaccination effort eradicated it, the first virus may have taken the lives of more people throughout history than any other infectious agent has. The second, for which there is yet no vaccine or cure, currently infects more than 47 million people.
These killers are variola and HIV, the viruses that cause smallpox and AIDS, respectively. Despite dramatically different ways of attacking the bodies they infect, the two viruses may have more in common than scientists have suspected.
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