1. Immune activity may be a cause of resistance to parasites, but it can also be a consequence of infection. Thus, the adaptive significance of an immune response is more accurately assessed when it is measured alongside both host fitness and infection status.
2. We sought to determine the significance of immune responses in a naturally coevolving host–parasite system in the wild, with support from laboratory experiments. We measured haemocyte numbers in Daphnia magna in relation to an infection that has a clear fitness consequence: infection with the bacterium Pasteuria ramosa causes sterilization.
3. Haemocyte number was consistently elevated in infected Daphnia in the field and in parasite exposed or infected hosts in the laboratory. Thus, elevated haemocyte numbers were essentially a symptom of infection, and we found no evidence that haemocytes help hosts exclude the parasite.
4. Consequently, these results provide an especially clear example where increased immune activity does not mean increased immunity or fitness: hosts with the highest haemocyte counts have extremely low health and low fitness potential.