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Local health officials are warning residents to cover up and use bug spray after state workers found mosquitoes with West Nile virus in Waltham. City officials were notified of the discovery July 18, and the community was sprayed for mosquitoes the following day as a precautionary step, Health Department director Walter Sweder said. He noted that while mosquitoes carrying West Nile are commonly detected in the area, no city residents have contracted the illness. According to the US Centers for Disease Control, 20 percent of people who contract the West Nile virus develop a fever, headache, body aches, joint pains, vomiting, diarrhea, or rash, but the symptoms usually respond to treatments. Less than 1 percent will develop a neurological illness; about 10 percent of those cases can be fatal, according to the CDC.