It makes them wheeze and sometimes means they find it hard to take part in sports.
But when the new vaccine was tested on people with asthma, the scientists at Imperial College in London found it made them feel better.
They gave them injections of tiny doses of the substance they're allergic to.
That stimulates the body's natural defences to get used to their allergy and therefore not get bad asthma any more.
It's not a cure, but it does stop them from getting severe symptoms.
The scientists are now trying to make the vaccine work for people whose asthma is caused by allergies to dust mites and pollen as well.
They hope it will be available within five years.