A team of researchers discovered that infections with two types of malaria parasite lead to greater health risks as one species helps the other to thrive.
The University of Edinburgh researchers found that one type of parasite leads to the second species being provided with more of the resources it needs to prosper.
The study in mice with equivalent malaria parasites showed that the body's response to the first infection produces more of the type of red blood cell that the second parasite needs.
In response to the first infection, millions of red blood cells are destroyed. The body responds by replenishing these cells. These fresh cells quickly become infected by the second type of parasite, making the infection worse, the team said.
The finding could explain why infections with both P. falciparum and P. vivax in people often have worse outcomes for patients than single infections, researchers added.
Researcher Sarah Reece said: "Immune responses are assumed to determine the outcome of interactions between parasite species but our study clearly shows that resources can be more important. Our findings also challenge ideas that one species will outcompete the other, which explains why infections involving two parasite species can pose a greater health risk to patients."
The study is published in the journal Ecology Letters.