Health officials at the U.S. Centers For Disease Control and Prevention in Atlanta (CDC) are monitoring a technician who inadvertently handled a live Ebola virus sample. The incident, which occurred Monday and was confirmed by federal officials on Wednesday, is the latest in a number of flubs placing the country’s top infectious disease control agency under scrutiny.

Meanwhile, the United Nations on Wednesday said a fourth U.N. staffer contracted the highly infectious virus in Liberia, one of the four West African countries where most of the 7,588 recorded fatalities have occurred since the latest Ebola outbreak began a year ago.  

Citing federal officials, the New York Times said Wednesday the CDC has confirmed that a live sample of the virus – rather than one containing the dead virus – was accidentally moved on Monday from one lab to another at the agency’s high-security facility in Atlanta. Less than a half-dozen people entered the lab before the mistake was discovered a day later, the CDC said. Attention has focused on one technician who handled the sample without full protective gear.

CDC Director Thomas R. Frieden faced a congressional grilling in July over his agency’s mishandling of anthrax samples in June. The mistake was similar in that technicians believed the samples they were handling were dead. In that case the equipment used to kill the anthrax spores failed to eradicate them. Dozens of employees were potentially exposed to the highly infectious spores, but none contracted illness.

Five thousand miles away in Liberia, a fourth international health emergency worker has contracted the Ebola virus, the United Nations Mission in Liberia (UNMIL) said on Wednesday. The announcement comes three months after two UNMIL health workers succumbed to the infection.

“The confirmation of an additional Ebola case in UNMIL at the start of the holiday period is a stark reminder that we must all remain vigilant until there are no cases in Liberia or West Africa,” the agency said.

Liberia, Sierra Leone and Guinea have been most impacted by the latest and most pronounced Ebola outbreak of the virus first described in 1976 near the banks of the Ebola River in the Democratic Republic of Congo.

Over 7,000 of the deaths in the recent yearlong outbreak have occurred in those three countries, while only one fatality has been recorded out of the region. The New England Journal of Medicine said in a report Wednesday analyzing the progress in the fight against the Ebola outbreak over the past year that increases in incidents have stopped in Sierra Leon and Guinea and that Liberia has reversed the spread of the virus. But, the report warned, the geographical pattern of the Ebola virus transmission is moving and getting broader, moving away from areas where health centers and proper burial practices have been established.