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The seasons appear to have a profound effect on how human genes work, according to scientists.

This may explain why some illnesses are aggravated in the winter, they say in Nature Communications, the BBC reports.

They found genes involved with immunity - the body's defence against infection - were more active in cold months.

And while this helps fight off viruses such as flu, it may trigger or worsen conditions, such as arthritis, where the body attacks itself, they say.

The international team of researchers analysed blood and tissue samples from more than 16,000 people living around the world.

Of the 22,000 genes they scrutinised - which is nearly all the genes humans possess - a quarter showed clear signs of seasonal variation.

The gene changes that interested the researchers the most were ones involved with immunity and, specifically, inflammation.

During cold, winter months - December to February for people living north of the equator and June to August for those in the southern hemisphere - these genes were more active.

When they studied people living close to the equator, where the temperatures are fairly high all year round, they noticed a different pattern. Immunity and inflammation was linked to the rainy season, when diseases such as malaria are more rife.

In Iceland, where it is cold most of the time, they found fewer seasonal changes.