Figure 2.

Parasite-dependent protective effect over age and time. Young children often have high levels of blood-stage parasitaemia. When these children are re-infected with sporozoites, little replication or development of parasites happens in the liver, inhibiting superinfection. The blood parasitaemia of an infection decreases with increasing age. Infected individuals become susceptible to further infections only once parasite densities fall below a critical threshold, raising the probability of superinfection.