A nasty fungus seems to have sex while it invades its human host.

The fungus causes chronic diarrhea in AIDS patients, organ transplant recipients and travelers. The finding could lead to the development of new treatments against these virulent pathogens.

Both sexual and asexual reproduction are common in fungi. Asexual reproduction means cells simply divide. Scientists were unsure which way this group of fungi, called microsporidia, reproduced. Now they know it's sexual.

"Microsporidian infections are hard to treat because until now we haven't known a lot about his common pathogen," said Soo Chan Lee, lead author of a new study and a postdoctoral researcher at Duke University in North Carolina. "Up to 50 percent of AIDS patients have microsporidial infections and develop chronic diarrhea."

"These infections are also detected in patients with traveler's diarrhea, and also in children, organ transplant recipients and the elderly," Lee added.

Of the more than 1,200 species of microsporidia, more than a dozen infect humans. Investigating the tiny fungi was difficult because they cannot live outside of an infected host cell and they have a small number of genes, which are rapidly evolving.

The Duke scientists used two genetic studies to prove that microsporidia are true fungi and that they evolved from sexually-reproducing fungi.

They also found that they are closely related to zygomycete fungi, another group that reproduces sexually, as well as asexually. The most familiar type of zygomycete is the fast-growing, white, fuzzy fungus that develops on strawberries and other high-sugar content fruits.

Lee and his colleagues found that microsporidia shared 33 out of 2,000 genes with zygomycetes that it doesn't share with other fungi. This genomic signature shows that the two types of fungi likely shared a common ancestor and are more distantly related to other known fungal lineages.

The study, funded by the National Institutes of Health and the Canadian Institutes for Health Research, also showed that microsporidia and zygomycetes have some of the same genes involved in sexual reproduction and that they're arranged in the same order in both groups.

These findings, detailed in the Oct. 30 issue of the journal Current Biology, show that microsporidia have a genetically-controlled sexual cycle and that they may be undergoing sexual reproduction while they're invading their host.

Lee said that the next step of the research is to explore the fungi's sexual reproduction, which may cause more severe infections because they use the host's cellular environment and machinery as a safe haven in which to reproduce.