(CNN) An outbreak of yellow fever has killed at least 146 people in Angola since December, according to the World Health Organization.
Most of the deaths have been in the bustling capital of Luanda, said Dr. Margaret Harris, a pandemic expert at the WHO. She described it as a "large outbreak."
The disease is transmitted by two types of mosquitoes, one of which is responsible for the Zika virus that has ravaged the Americas.
The yellow fever virus is transmitted when a mosquito bites an infected monkey and then bites a human being.
Symptoms include fever, muscle pain, nausea, vomiting and loss of appetite, and can be confused for malaria or other mosquito-borne illnesses.
A small percentage of infected people experience a second phase within 24 hours of becoming ill. It comes with more advanced symptoms, including jaundice, hemorrhaging and bleeding from the eyes, nose and mouth.
At least half of the patients who get the second phase of the disease die within 10 to 14 days.
There is no treatment for yellow fever, but patients can get supportive care.