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As if disease-carrying mosquitoes weren’t bad enough in the first place, scientists have found the pesky insects may be able to infect us with two viruses at the same time.

A small lab study that exposed mosquitoes to blood infected with varying combinations of dengue, Zika and chikungunya proved that the mosquitoes were able to pick up two viruses — Zika and chikungunya — at the same time.

The results suggest that human beings could be infected with both the Zika virus and the chikungunya virus in one bite, but researchers need to conduct more studies to find out how this affects a person’s health and immunities to future disease. The research follows a previous experiment that found mosquitoes can also carry dengue and chikungunya at the same time.

“We didn’t know what to expect, and now we know that [Aedes aegypti mosquitoes] are very susceptible to co-infection and can co-transmit at a relatively high frequency,” said lead investigator and Colorado State University researcher Claudia Rückert, who presented the study results at the 2016 Annual Meeting of the American Society of Tropical Medicine and Hygiene on Monday. “This could mean that there are more co-infected patients than we thought.”

Scientists in Nicaragua recently estimated that one in five people who tested positive for one of the three viruses also had at least one of the remaining two.

The question of simultaneous infection in both mosquitoes and humans is a vital one, as the viruses circulate in the same populations and can confuse diagnostic tests and delay treatment. For instance, patients who come down with fever, rash and joint pain in Brazil aren’t quite sure if they’ve got Zika, a relatively mild disease that has devastating results for a developing fetus, or chikungunya, which carries severe, disabling symptoms, or dengue, which is incredibly painful and potentially fatal.

There is no vaccine or cure for these viruses, and the treatment for all of them is the same: The only thing that people can do to get better is to rest, drink fluids and take fever and pain medication. However, being able to accurately pinpoint Zika virus, for example, could help pregnant women make faster decisions about prenatal care. But as things stand, diagnostic tests struggle to tell the difference between the three viruses.

Some researchers are also investigating the possibility that the viruses can compete with each other ― or exacerbate each other’s symptoms ― if one person gets them at the same time. Researchers from Emory Vaccine Center have found that people who had dengue virus develop antibodies that react with Zika virus, but they aren’t sure whether those antibodies neutralize Zika or enhance it’s effect on human cells.

One study presented at the same conference found that two patients who had tested positive for multiple viruses also developed a rare disorder called “dancing eyes-dancing feet syndrome,” a condition that causes rapid eye movements and muscle contractions. This could suggest, said Rückert, that co-infections may be linked to serious complications beyond the symptoms of any individual virus.

Another theory on the effect of co-infections is that dengue-Zika interactions could be the reason Brazil was hit particularly hard by cases of congenital Zika syndrome, a constellation of symptoms a baby is born with if they are infected with Zika in the womb. However, this is just one theory among many that could explain why some countries’s newborns were hit harder by congenital Zika syndrome than others.

As scientists continue to sort this out, it’s clear that disease-carrying mosquitoes can be really bad news in countries where serious viruses are circulating. The takeaway for readers, Rückert says, is that mosquito bites may carry not just the risk of one disease but several, and that these co-infections may stress your immune system more than just a single virus.