When 300 people with the disease were tested, it turned out that those with very bad asthma were very different to people with the weaker kind.
The weaker kind asthma can be easily controlled by medicines called corticosteroids.
But the worse kind of asthma is very hard to treat this way.
The two groups also had very different amounts of important chemicals in their blood.
They also found that women are four times as likely as men to have serious asthma.
New medicine
Now scientists understand the differences between the two types of asthma, they could develop new and better ways of treating it.
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