The World Health Organization reported Friday that nearly 200 people had been in contact with a 17-year-old in Liberia who was confirmed to have died of Ebola on June 28, the country’s first Ebola case since May 9, when it had been officially declared free of the highly contagious disease. In an assessment posted on its website, the W.H.O. said that all of the contacts of the teenager were being closely monitored and that two of them had already tested positive for the virus, confirming what the Liberian health authorities had reported this week. The organization also said it was dispatching epidemiologists and other specialists to ensure that the victim’s hometown “is fully involved in identifying contacts and preventing any further disease spread.”