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"The virus may be bumping into people it can't infect any more." Latest World Health Organization data show new cases of infection in West Africa's unprecedented Ebola epidemic dropping dramatically in Guinea, Sierra Leone and particularly in Liberia. Most experts are sure the main driver is better control measures reducing direct contact with contagious patients and corpses, but there may also be other factors at work. So-called herd immunity is a feature of many infectious diseases and can, in some cases, dampen an outbreak if enough people get asymptomatic, or "sub-clinical" cases and acquire protective antibodies. After a while, the virus - be it flu, measles, polio - can't find non-immune people to be its hosts. But some specialists with wide experience of disease outbreaks are highly sceptical about whether this phenomenon happens in Ebola, or whether it could affect an epidemic. "There is some suggestion there may be cases that are less severe... and there may even be some that are asymptomatic," said David Heymann, an infectious disease expert and head of global health security at Chatham House. "But herd immunity is just the wrong term.