An American health care worker infected with the ebola virus disease while working in West Africa was transferred to the U.S. last Friday. The unidentified patient has been downgraded to critical condition as of Monday, according to doctors at the U.S. National Institutes of Health (NIH). Treatment is taking place at the NIH Clinical Center in Bethesda, Md., a high-level containment facility.

The patient was working with Boston-based Partners in Health. According to the U.S. Centers for Disease Control and Prevention (CDC), 11 other workers from the same organization are now in the U.S. where they were brought for monitoring. All were working in Sierra Leone, which is one of the three West African nations most severely affected by the ebola outbreak that started late in 2013.

The patient was flown from Sierra Leone on a chartered plane that was equipped with an isolation unit. No other information has been released about the health care worker’s identity. This patient is the second with ebola to be treated at the NIH. The other workers who have returned to the U.S. are isolating themselves for 21 days, being monitored near hospitals at NIH, Nebraska and Emory. One has begun to show symptoms and has been hospitalized in a Nebraska hospital isolation unit as a precaution. None have tested positive for the virus.

The CDC is continuing to trace contacts of the NIH patient who may have been exposed to the virus, which has killed more than 10,000, mostly in the West African nations of Guinea, Liberia and Sierra Leone. More than 24,000 have been infected with the virus since the outbreak began in West Africa in December 2013.