The World Health Organization (WHO) said Thursday that the worst yellow fever outbreak in decades has killed 178 people in Angola. The WHO said in a statement that more than 450 people have been infected by the disease since the outbreak was first reported in December last year. The outbreak spread from the capital Luanda to six of the country's 18 provinces. The WHO said that while 5.7 million people have been vaccinated against yellow fever, there is a global vaccine shortage. Half of the severely infected patients die within 10 to 14 days if they do not receive treatment. Yellow fever is spread by the same species of mosquito that spreads the Zika virus.