The Conversation has reported that a new study by researchers at the University of Bath, published in Evolution Letters, shows parasites can readily evolve different mechanisms to defend their hosts from other infections, which suggests that host protection should be common in nature.

The natural world is full of examples where parasites are harmful under some conditions and helpful under others. Bacteria living in peoples guts can occasionally cause problems, but they also prevent colonisation by more harmful microbes such as Salmonella enterica, which causes food poisoning. Similarly, bacteria that commonly infect insects are usually costly but can provide protection against more deadly infections. And the larvae of monarch butterflies are more likely to survive infection by a parasitic fly when they are also infected by a protozoan.

Parasites can also help their hosts in other ways, for example by causing more serious disease in other species. This is one of the main reasons why grey squirrels have rapidly displaced red squirrels from most of the UK. Grey squirrels are carriers of squirrel pox virus, which is usually fatal to red squirrels but is rarely harmful to greys. Likewise, some species of bacteria engage in a form of primitive biological warfare by carrying viruses to which competing bacteria are not immune.

Recent lab experiments have shown that mildly harmful bacteria living inside microscopic worms can evolve in just a few days to protect their hosts from a lethal infection. This striking result indicates that bacteria can rapidly evolve host protection against other infectious diseases.

Very little is known about how and when such evolution occurs in nature, and whether if a parasite evolves to protect its host from a more deadly infection this means the enemy has now become a friend.

Using mathematical modelling, the researchers explored the evolution of two forms of host protection; resistance and tolerance. Parasites that protect by conferring resistance to their hosts reduce the likelihood that a second species will be able to infect them, such as when bacteria in the gut prevent colonisation by other microbes. In contrast, parasites that confer tolerance to their hosts reduce the harm caused by another species after it infects them, as appears to be the case with the protozoa that protect monarch butterfly larvae from parasitic flies. The team discovered that both forms of host protection evolve under a wide range of conditions even though the protective parasite may have to divert resources from its own growth or reproduction to defend the host. Protection still evolves because this cost is more than offset by the increased survival of the host, and hence the protective parasite.

There are some notable differences between the two forms of protection. For instance, resistance usually increases the population size of the host, but tolerance can have a negative effect because it increases the overall prevalence of disease. These differences indicate that the mechanism of protection is crucial for determining whether a protective parasite is truly beneficial.

The researchers can now combine mathematical modelling with lab experiments of evolving microbes to answer intriguing questions about how other species evolve in response to host protection. For example they can look at whether the host evolves to harbour the protective parasite, and whether this is how humans developed a symbiotic relationship with some of their gut bacteria. They can also look at whether more harmful parasites evolve to overcome host protection. Answering questions like these can help find new ways to treat infectious diseases.