(CNN) -- Uganda is planning to inoculate 2.5 million residents in the East African nation's northern districts, where a yellow fever outbreak has killed 45 people and sickened another 183.
The outbreak began in early November but was not confirmed as yellow fever until December 23 by the U.S. Centers for Disease Control and Prevention, Dr. Nathan Kenya-Mugisha, director general of the Ugandan Health Ministry, said Tuesday.
He said the cause of the outbreak was not known but that it could have originated in animals.
"We are doing all we can to contain the disease," Kenya-Mugisha said. "We are working with our partners to mobilize our resources to vaccinate (in) the affected areas."
Yellow fever is a viral disease transmitted to human beings through the bite of infected mosquitoes. It can lead to severe hepatitis and hemorrhaging, according to the CDC.
There is no specific treatment for yellow fever, and care is based on symptoms that include fever, backache, nausea and vomiting. People can pass on the illness without ever having its symptoms.
The World Health Organization estimates there are 200,000 cases of yellow fever and 30,000 deaths worldwide each year. Uganda's last outbreak of yellow fever, a disease endemic to tropical regions, was in the early 1970s, Kenya-Mugisha said.
CNN's Salma Abdelaziz contributed to this report.