There is no specific treatment for yellow fever, which is largely transmitted in urban settings by mosquitos, yet vaccinations are nearly 100% effective. Recent decades have seen the introduction of mass vaccination programmes to combat the disease. But in recent years, cases have begun to rise. Before 2017, there had not been an outbreak of yellow fever in Nigeria for more than 20 years, said Adesola Yinka-Ogunyele, a doctor and epidemiologist at the NCDC. The climate crisis, increased migration and fewer vaccination campaigns are among the reasons for the increase even before the coronavirus pandemic.