Catching flu early in life may actually increase the chances of a child developing asthma later, say experts.
House dust mite faeces can trigger asthma
Their finding, in a study of mice, contradicts the suggestion that early infections have a protective effect.
Writing in the journal Nature Immunology, the team from Stanford University say that flu boosted the body's allergic responses.
Rising asthma and allergy rates in the western world remain a puzzle to experts in respiratory medicine.
In recent years, the "hygiene hypothesis" has been put forward.
This suggests that cleaner conditions inside modern houses means that early in life, babies are not exposed to as many bacteria, viruses and also potential future allergy-causing substances such as house dust mite faeces.
While ordinary logic states that it is a good thing for a parent to protect a vulnerable baby from such things, many scientists believe that, at key moments in early childhood, exposure is required to help programme the immune system to respond properly.
Asthma and allergy are an inappropriately powerful immune response to triggers which in fact are not a direct threat - such as fur, pollen and dust mite waste.
However, the latest research from the US adds to the debate on the subject.
The team, led by Professor David Lewis, exposed mice to flu, then measured the levels of key chemicals that show how the immune system has responded to the infection.
They expected to find that the illness would lead to reduced immune responses to common allergy-causing substances - adding weight to the theory that the infection offered protection.
However, the effect was precisely the reverse - having the flu actually boosted subsequent asthma symptoms produced by these allergens.
The scientists believe that immune cells and chemicals produced in response to flu hang around in the lungs afterwards, producing the unwanted increase in inflammation associated with asthma and wheezing.
Dr John Harvey, from the British Thoracic Society, said that it was important to remember that the emergence of asthma was probably linked to a number of factors, not least the genetic makeup of the child involved.
He said that research suggested that the timing of exposure to infections and allergens might be key to the eventual outcome.
He said: "One interesting project at the moment is to take parents who are both asthmatic or suffer from allergies, and when they have children, in the final three months of pregnancy, to reduce exposure, as much as possible, to asthma triggers such as house dust mite.
"There is some hope that doing this might offer some protection."