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GENEVA (Reuters) - Nigeria and Senegal could be declared Ebola-free within days after completing a 42-day period with no new cases, the World Health Organization said on Tuesday. "If the active surveillance for new cases that is currently in place continues, and no new cases are detected, WHO will declare the end of the outbreak of Ebola virus disease in Senegal on Friday 17 October," the WHO said in a statement. For Nigeria, the date is next Monday, Oct 20. Senegal had one patient who was confirmed to have Ebola but he recovered and appears not to have infected anyone else. In Nigeria, one traveller from Liberia triggered an outbreak in which eight people died, most of them health workers, before it could be contained. But in the three worst affected countries, Liberia, Sierra Leone and Guinea, "new cases continue to explode in areas that looked like they were coming under control," WHO said. "An unusual characteristic of this epidemic is a persistent cyclical pattern of gradual dips in the number of new cases, followed by sudden flare-ups." WHO says that waiting for 42 days from the time when the last person with high risk exposure tests negative for the virus gives sufficient confidence to declare an outbreak over. The 42-day period is twice the generally accepted maximum incubation period of the virus. However, some incubation periods are longer - that WHO said that in 95 percent of cases the incubation period was between one and 21 days. In 98 percent it was no longer than 42 days. The health agency has said that the virus can survive even longer, remaining for as much as 90 days in the semen of an infected man. Outside West Africa, the spread of the disease has been confirmed in Spain and the United States. Possible cases have been investigated in several other countries, but none has yet turned out to be Ebola. The WHO said it was concerned by media reports that some countries facing a possible first Ebola case had declared the cases to be negative within hours. "Such rapid determination of infection status is impossible, casting grave doubts on some of the official information that is being communicated to the public and the media," it said. Countries without recognised laboratories specialising in viral haemorrhagic fever testing should send their first 50 negative specimens to a WHO collaborating centre, it said. All countries should have their first 25 positive tests double-checked.