People who have caught hepatitis A may be protected against asthma, according to scientists in the United States.
In the UK, 8m people have been diagnosed with asthma
They believe the virus stops the immune system from reacting to allergens that may trigger asthma in some people.
However, writing in the journal Nature they said only people with a particular version of a key gene may be protected.
Nevertheless, with rates of hepatitis A falling they say it may explain why so many people are developing asthma.
Until about 1970, most people living in the West carried hepatitis A antibodies, which means that at some point they had been infected by the virus.
Hepatitis A affects the liver and can be spread through poor personal hygiene and sanitation.
In many cases, the symptoms are so mild people do not even realise they are infected.
In others, it can cause fever, tiredness, loss of appetite, nausea, abdominal discomfort, dark urine, and jaundice. These symptoms usually last less than two months.
However, improvements in living standards and hygiene have seen rates of hepatitis A infection drop in recent decades.
Now just one in four people living in the West show signs of having been exposed to the virus.
In contrast, rates of asthma have almost doubled over the same period. In the UK, 8 million people have been diagnosed with the disease.
Scientists are unsure why this has happened but there is speculation that it may be related to improvements in hygiene.
But Dr Dale Umetsu and colleagues at Stanford University in California have now suggested that there could be a link with the incidence of hepatitis A.
They have found that the virus attacks the body by entering immune cells through a receptor called TIM-1. This receptor is regulated by the TIM-1 gene.
The scientists believe that hepatitis A is able to enter immune cells more efficiently in people with an altered version of this gene.
They have also suggested that this efficient infection stimulates the immune system and prevents it from reacting to allergens in the future.
Allergens, like dust and pollen, are believed to trigger asthma in some people.
The scientists said further research is needed to confirm their theory and to find out if the timing of hepatitis A infection is important.
For instance, if infection in childhood is important to reduce the risks of developing asthma.
Professor Martyn Partridge, chief medical adviser at the UK's National Asthma Campaign, welcomed the study.
"This latest study may help us understand why infections, such as hepatitis A, may have a protective effect against the development of allergies and asthma.
"The goal would be to find a suitable vaccine to provide the beneficial effects of such early life infections.
"However, even if we had such a 'controlled infection' or vaccine we don't know at what time of life it should be given for maximum effect."