ZMapp, the drug that has been used to treat seven patients during the current Ebola epidemic in West Africa, can completely protect monkeys against the virus, research has found. The study, published online today in Nature, comes the day after the World Health Organization warned that the Ebola outbreak, which has killed more than 1,500 people, is worsening and could infect 20,000 people before it ends. A fifth West African nation, Senegal, reported its first case of the disease on Friday. UTMB’s Thomas Geisbert estimates that day 5 of infection in the monkeys studied is roughly equivalent to days 7 to 9 of a human infection. People can develop symptoms up to 21 days after they contract Ebola, although signs commonly develop between 8 and 10 days after infection. The study authors say that ZMapp works in an “advanced” stage of the disease. The news also appears in the Los Angeles Times, NBC News, Bloomberg, Washington Post, Scientific American, Reuters and Infection Control Today, among hundreds of other outlets.