Men represent the majority of coronavirus cases so far, according to a new study. Researchers have a few guesses as to why.

Wuhan coronavirus

The deadly coronavirus that originated in Wuhan, China, has infected more men than women, and scientists are divided about why that is.

The virus has killed more than 910 people and infected more than 40,000, with the vast majority of cases in mainland China. (For the latest case total and death toll, see Business Insider's live updates here.) A recent study of nearly 140 coronavirus patients at a Wuhan University hospital offers one of the broadest pictures of how the virus operates in humans so far.

The researchers found that the virus is most likely to affect older men with preexisting health problems. More than 54% of the patients in the study were men, and patients had a median age of 56.

Other recent studies have produced similar results. A study of 99 coronavirus patients at Wuhan Jinyintan Hospital found that the average patient was 55.5 years old, and men represented around 68% of the total cases. A third study of nearly 1,100 coronavirus patients (which is still awaiting peer review) identified a median age of 47, with men representing around 58% of all cases.

This data has led some researchers to suspect that men have certain biological conditions that make them more susceptible to the virus. But other researchers aren't so sure.

SARS affected mostly men, too

In the absence of much reliable, broad data about the new coronavirus, scientists have turned to a similar outbreak - the SARS pandemic from 2002 to 2003 - for clues.

SARS was also a coronavirus that jumped from animals to people in wet markets. It shares about 80% of its genome with the novel coronavirus, and like the current outbreak, it infected more men than women.

Customers wearing face masks shop inside a supermarket following an outbreak of the novel coronavirus in Wuhan, Hubei province, China February 10, 2020. China Daily via REUTERS

In 2017, researchers at the University of Iowa set out to investigate why that was by infecting male and female mice with SARS. Mice studies don't necessarily have definitive implications for humans, but the researchers did find that male mice were more susceptible to the virus than female mice.

The team attributed those results to genes located on the X chromosome, as well as hormones such as estrogen, that may keep a virus from spreading throughout the female body.

The researchers at Wuhan Jinyintan Hospital gave a similar explanation for why more of their coronavirus patients were men, suggesting that women may have a "reduced susceptibility" to viral infections. But they also noted that many patients with severe cases had chronic illnesses such as cardiovascular disease or diabetes. Those particular illnesses tend to affect middle-aged men more than middle-aged women.

The outbreak started among mostly male workers

Aaron Milstone, an epidemiologist at Johns Hopkins University, told Business Insider that he doesn't think one subset of the population is necessarily more vulnerable than another.

"When we see any new virus, the whole population is susceptible," he said.

The new study from Wuhan University also found that the share of male and female coronavirus patients in the ICU was about the same as the share of male and female coronavirus patients in other parts of the hospital. That suggests the men's symptoms weren't more severe than women's overall (though some patients are still hospitalized, so their conditions could change over time).

The researchers attributed the higher number of male patients to the fact that the outbreak originated at the Huanan Seafood Wholesale Market in Wuhan, among mostly male workers there.

But Milstone said any data set from this coronavirus outbreak is still inherently limited at this point.

"There can be underreporting because of under-testing, especially because this is not a commercially available test," Milstone said. "We don't know enough yet about what's happening."

Have you been personally impacted by the coronavirus epidemic? Is your city or community on the front lines of this disease? Have you or someone you know been tested or diagnosed? We want to hear your story. Please email science+coronavirus@businessinsider.com.

Read more about the coronavirus:

Only a handful of children have been diagnosed with the coronavirus - and experts have a few guesses as to why

How deadly is the new coronavirus? Research so far suggests the fatality rate could be low.

Everything we know about the pangolin - the scaly mammal that may have spread the coronavirus to humans

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