In its original jungle cycle, the mosquito sucks the blood of an infected monkey. The mosquito develops a permanent infection, in which the virus accumulates in its salivary glands. Then the mosquito bites another monkey, which then also becomes infected with the virus. A person travelling through the jungle may also become infected by an infected mosquito. When this person returns to urban areas, a new cycle begins.
Urban cycles start when an infected traveller returns from the jungle.
A mosquito bites the traveller, who then becomes infected and passes the virus on to other people, and either an epidemic breaks out, or an endemic situation is perpetuated. In addition, there is an increasing number of epidemics, in which a large number of people suddenly develop yellow fever. Every year about 200,000 cases of yellow fever are recorded, and 30,000 of these die, but the figures are underestimated because of poor record-keeping. In total, yellow fever occurs in 33 countries and 468 million people are at risk of catching the disease. As yet there is no yellow fever in Asia, but it is feared that the high level of international travel could introduce the virus by means of infected people.