Multiple invasions (MIs) or infections, i.e. those by more than 1 parasite in the same erythrocyte, could be the result of parasite density or, alternatively, to parasite-related factors or host-related factors. According to the last possibility, to our knowledge, only 3 laboratory studies of malaria have found an increase in the occurrence of MIs when antibodies to parasite antigens were present. Therefore, we tested the possibility that MIs were influenced by the host immune status, using as model the malaria-like parasite Haemoproteus infecting blue tits (Cyanistes caeruleus). Avian hosts infected with Haemoproteus were medicated with primaquine or injected with saline solution and the density of infection and the presence of MIs counted. Medication treatment reduced significantly the density of infection by Haemoproteus in females but not in males. For females, the presence of MIs was positively associated with both the density of infection and the immunoglobulin levels on each capture, but no association was found between the treatment and the presence of MIs. For males, the density of infection but not the immunoglobulin levels was positively associated with the presence of MIs. In addition, medicated males supported more MIs than controls. Our results represent the first line of evidence in the wild for a possible role of the host immune system promoting MIs.