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By Diadie Ba and Saliou Samb GENEVA/LAGOS (Reuters) - The West African state of Senegal became the fifth country to be touched by the world's worst Ebola outbreak on Friday, while riots broke out in neighboring Guinea where infection rates are rising fast. At least 1,550 people have died of Ebola and more than 3,000 have been infected since the virus was detected in the remote jungles of southeastern Guinea in March, and quickly spread across the border to Liberia and Sierra Leone. The World Health Organization warned on Thursday the actual number of cases could be up to four times higher and said that a total of 20,000 people could be infected before the outbreak ends. In the Guinean city of Nzerekore, riots broke out on Thursday night over rumors that health workers had infected people with the Ebola virus, a Red Cross official and residents said.