Sex was once thought to be the domain of higher life forms. But now a common fungus -- one that causes deadly infections in humans -- appears to reproduce sexually.

The fungus, aspergillus fumigatus, has also been linked to asthma. Scientists always thought it reproduced asexually, a method of simple cell division used by many microbial creatures.

A new study finds the fungus has a series of genes required for sexual reproduction. An analysis of 290 specimens revealed nearly equal proportions of two different sexes or 'mating types,' which in theory could have sex with each other, the researchers said.

That might provide clues for treating people infected by the fungus.

"The possible presence of sex in the species is highly significant as it affects the way we try and control disease," said David Denning of the University of Manchester. "If the fungus does reproduce sexually as part of its life cycle, then it might evolve more rapidly to become resistant to antifungal drugs -- sex might create new strains with increased ability to cause disease and infect humans."

The discovery, announced today, is detailed in the journal Current Biology. The work was led by Paul Dyer at the University of Nottingham.

The study also found that genes had been, or were being, exchanged between individuals of the fungus and that some key genes involved with detecting a partner were active in the fungus.

"Taken as a whole, the results indicate that the fungus has a recent evolutionary history of sexual activity and might still be having sex so far 'unseen' by human eyes," Dyer said.

An offbeat thought: "The fungus is very common in compost heaps so these might be a hotbed of fungal sex," Dyer said.