Infants, young children and travellers from areas without malaria are more susceptible to malaria because they lack the acquired immunity that can develop over time in those living in endemic regions. Children who survive repeated exposures are eventually able to control malaria better: older children and adults can still become infected, but parasite burdens are lower and the infections rarely cause symptoms or serious illness.

However, immunity can be lost when a person moves away from an endemic region. Likewise, people with advanced HIV disease, other forms of immune suppression and pregnant women sometimes lose their immunity to malaria.

How frequently people are exposed to malaria determines how soon they acquire immunity, but it also can determine the clinical pattern of illness in a population. In regions of Africa where there are high rates of transmission, people can be exposed to hundreds of infective bites each year, with up to five infective bites per night during periods of peak transmission^[1][2]. Where exposure is so intense, surviving children are likely to have partial immunity before the age of five, and severe malaria among older children and adult will be rare. Where exposure is less frequent, people do not tend to build up immunity, so the infrequent cases of malaria that occur are often severe, and similar to the pattern of disease seen in travellers without acquired immunity.