Nigeria now has 18 Ebola cases, after a fourth case surfaced in Port Harcourt, home to Africa's biggest oil and gas industry, the health minister said on Wednesday.

The Ebola outbreak in Africa's most populous country began on July 20 when a Liberian man with the disease collapsed at Lagos airport, spreading the virus to the hospital staff who treated him.

A man who had traveled with him then skipped quarantine and traveled to Port Harcourt, bringing the disease there. A doctor died in the oil city last week. Health Minister Onyebuchi Chukwu said a patient he had treated had also now died.

A total of 255 people were under surveillance in Port Harcourt, while 41 were in Lagos.