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.

She said those with the vulnerability have a overblown immune response.

"Some people will experience more severe symptoms than others when they are acutely sick with the same infection because their body's response is more intense which in turn is due to their genetic make-up," Vollmer-Conna said.

She said: "This group in the population were found to spend twice as many days in bed during the acute illness and they also reported more than twice as many days when they were unable to perform their normal roles and duties."

The findings could ultimately help identify people who are vulnerable and give them individualised prevention and treatment for common infectious diseases, and priority therapy in the event of a pandemic.

"In certain conditions, it may even be possible to save lives," she said.