As it turns out, "man flu" is a real thing.

Scientists from Harvard University found that women's sex hormones give them a stronger immune system than men, making men more likely to catch the flu.

After studying mice, the scientists found that women's resistance to the flu was linked to the enzyme nitric oxide synthase 3 (NOS3), which is activated by the release of the female sex hormone oestrogen, reports Daily Mail. 

Once they tried deleting the NOS3 gene that is linked to the female sex hormone, women lose their resistance to the virus. 

One way to get men on an equal playing field as women when it comes to resistance to the flu is by increasing their levels of the female hormone oestrogen, that is, if they don't mind the side effects, reports METRO. 

Scientists hope that these findidngs will help them, among other researchers, enhance the human resistance to both common and serious lung infections, reports Daily Mail.