Everybody knows antibiotics don’t work on viruses, right? Not, it turns out, if you’re a female mouse with a nasty case of genital herpes, in which case a well-timed dose of antibiotic might be just the thing, according to new research published in the journal Nature Microbiology.

The researchers, led by Akiko Iwasaki from the Department of Immunobiology at Yale University in New Haven, US, infected mice with the herpes simplex virus – no trifling matter in rodents. The virus migrates from the vagina to the spinal cord resulting in hind limb paralysis, hair loss and, in some cases, death.

However, mice that were pre-treated with the antibiotic neomycin, used in humans to treat ear and skin infection and to sterilise the bowel before gut surgery, were largely spared this fate. They displayed, write the authors, “little to no disease pathology”.

How could antibiotics, which the received wisdom says are only effective against bacteria, possibly kill viruses?

Neomycin, it seems, is able to hack into the body’s virus-slaying mechanism by recruiting dendritic cells, key regulators of the immune system. The result is a bumping up, by as much as two-to-fivefold, of the expression of genes stimulated by the immune protein interferon, which produce a range of virus-killing substances.

And the good news isn’t limited to herpes.

A shot of neomycin up the nose was able to ward off influenza A in 40% of mice, also by boosting those interferon-stimulated genes, this time in the lung. On top of that, the researchers found that kasugamycin, an antibiotic belonging to the same class as neomycin, shut down replication of the devastating mosquito-borne virus known as zika, linked to stunted head and brain growth in babies of women infected during pregnancy.

The study included a very important check. The team made sure their antibiotics weren’t simply knocking out some of the legions of bacteria camped throughout the body, especially in the gut, collectively called the microbiome.

It is increasingly understood that our own special mix of gut microbes has serious clout in how well the immune system handles disease. A study published in Science in early 2018, for example, showed that people whose gut bacteria were depleted by antibiotics did worse on treatment for lung and kidney cancers, an effect related to impaired immunity.

Iwasaki’s team repeated their experiment in “germ-free” mice, which have no such bugs growing in or on them. The antiviral effects held all the same, showing they occur independent of whatever bacteria happen to be in residence at the time.

If you find yourself with the sniffles or an unpleasant itch, however, don’t expect to be racing off to the doctor for antibiotics any time soon. Note that most results were in mice treated before the infection began, although there was some success treating herpes in mice given the antibiotics four hours after infection.

Nonetheless, isolating the mechanism by which an antibiotic might treat viruses is a big step and could, write the authors, “be useful for the future design of novel broad-acting antivirals”.