Children who are exposed to infection from a younger sibling may be better able to fight off multiple sclerosis later in life, a new study suggests.

According to the "hygiene hypothesis," exposure to infections early in life from household dust or germy siblings and surfaces may reduce the risk of developing certain diseases in adulthood.

It's thought that improvements in sanitation and health care in the developed world may be linked to the increasing prevalence of multiple sclerosis.

In the autoimmune disease MS, scientists say the body's overactive immune system attacks the protective sheath around nerves that conduct electrical signals.

Multiple sclerosis attacks the nervous system, causing paralysis and sometimes blindness.

Until the age of two, children cough and sneeze their way through many common viral infections. The hygiene hypothesis suggests that exposure to infections passed from younger brothers and sisters may help to develop the immune system of older children in the family.

Researchers set out to test the hypothesis by comparing 136 adults in Tasmania who have multiple sclerosis with 272 people who don't. They were all interviewed about their childhood environment.

In the Jan. 26 issue of the Journal of the American Medical Association, the investigators report those who had up to five years of contact with a younger brother or sister had an 88 per cent reduced risk of suffering MS.

Among adults who lived with a young sibling for one to three years, a 43 per cent reduced risk was observed.

People who were exposed to a younger sibling for longer seemed to have a lower risk of developing mononucleosis or an overactive reaction to Epstein-Barr virus, antibodies from blood tests taken for the study showed.

If people aren't exposed to Epstein-Barr virus until their teens or later, it tends to cause mononucleosis at least 35 per cent of the time, according to the U.S. Centers for Disease Control.

Study author Anne-Louise Ponsonby of the Menzies Research Institute in Hobart, Australia, and her colleagues note their findings need to be confirmed by further studies.