Blame your genes if next time flu lay you down for weeks.

A study has found genes responsible for the less or high immunity of people as some recover from the flu overnight while some can take weeks to recover, ABC report said.

Experts in Sydney have discovered that people who carry certain high risk genes are eight times more vulnerable to a severe and prolonged illness when they have an infection.

A smaller group of people have a genetic combination that makes them particularly hardy, with a less severe illness.

"We all know that when people get sick some take a long time to recover and while others seem to get over it very fast, and what we've been able to show are the possible genetic reasons for this," Ute Vollmer-Conna of NSW University said.

The study, published in a journal Clinical Infectious Diseases, is the first to explore the genetic determinants of the severity of sickness.

It analysed differences in immune response among 300 people diagnosed with acute glandular fever, Ross River virus or Q fever infections in the central western NSW town of Dubbo.

Researchers looked at the genetic variants of five cytokines, protein hormone messengers of the immune system which defend against infection. About 28 per cent had the genetic susceptibility to more severe and prolonged illness, while 18 per cent had protective genes.

But it was too soon to say if the general population was affected to the same extent, Ute Vollmer-Conna said.

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