The World Health Organization declared Liberia free of Ebola for a second time on Thursday, several months after it was first thought to have halted the virus in May, only to experience a brief resurgence of cases that puzzled epidemiologists. “Today the World Health Organization declares Liberia free of Ebola virus transmission in the human population,” said Dr. Alex Gasasira, the organization’s representative in Liberia. But the Ministry of Health called for 90 days of “heightened surveillance” around the country to watch for any new cases. Dr. Gasasira applauded the government and Liberian communities for their quick response to the May outbreak, which left two people dead. The government has urged all Liberians to continue observing preventive measures like washing hands and is strengthening its monitoring efforts across the country, said Dr. Francis Ketteh, Liberia’s chief medical officer. More than 4,800 people have died in Liberia because of the Ebola crisis, according to the World Health Organization.