Tiny parasites such as bacteria and viruses may battle for control of precious bodily resources, but they can also open the door for additional invasions by their "friends." A new study finds that interactions among many parasites in the same host body can dramatically boost chances for new infections or, in other cases, lessen the odds.

Not only did the parasites in the study all interact with each of the other parasites, but they also varied wildly in terms of impact within the mouse-like voles that acted as hosts – generating five times greater risk of infection in one case involving two parasites, and a reduction of infection risk to just 15 percent in another case. [The 10 Most Diabolical and Disgusting Parasites]

This marks the first long-term study to track parasite interactions within the same individuals in the wild, so that researchers could rule out the effect of other factors, such as seasonal changes. Voles proved ideal in this case, because of their short life spans that range from just 3 to 6 months.

"Because we followed a large number of individual animals over the course of their lives, we were able to pinpoint the time an animal acquires an infection and examine what factors influence susceptibility," said Sandra Telfer, a parasite ecologist at the University of Liverpool in England.

Researchers drew blood samples from almost 6,000 voles about every four weeks, using four different populations over several years. They focused on cowpox virus and a protozoan, or single-celled, parasite called Babesia microti, along with species of Bartonella bacteria and Anaplasma phagocytophilum (a tick-transmitted parasite to humans and other animals).

The huge impact on infection chances rivaled more traditional factors, such as season. For instance, infection risks from certain parasites can change during the course of a year because of their seasonal life cycles.

"The size of the effects was perhaps the most surprising," Telfer said in an e-mail. "The impact of other parasites on infection risk was essentially as significant as season."

Past interactions among bacteria or viruses – classified as microparasites – are not unexpected. One well-known example is the HIV virus, which cripples the human immune system's defenses and opens the door for other infections.

But researchers have typically focused on single parasites that cause recognizable disease symptoms, or creepy effects, such as mind control. They have paid less attention to the more invisible web of interactions among parasites within the same host.

A renewed look at the community of parasites living in a host may change how experts decide to tackle infectious diseases. For instance, a well-intentioned health policy that tries to eliminate a certain parasite might just leave people vulnerable to invasion by a worse parasite.

"If some human parasites are equally interactive, our current, disease-by-disease approach to modeling and treating infectious diseases is inadequate," said Kevin Lafferty, a parasite ecologist at the University of California in Santa Barbara, in a perspectives article on the new study in the Oct. 8 issue of the journal Science.

As for Telfer and her colleagues, they hope to next study the interactions between tiny microparasites and larger macroparasites, such as nematode worms. Their research will still focus on voles, but Telfer has urged new studies that look at co-infection in humans.

"To improve our ability to predict and control disease in humans we need to study parasite communities, rather than parasites in isolation," Telfer said.