Asthma causes breathing problems. These breathing problems are called attacks or episodes of asthma. Doctors are not exactly certain how you get asthma, but they do know that once you have it, your lungs react to things that can start an asthma attack. Asthma attacks may start suddenly. Or they may take a long time, even days, to develop.
Attacks can be severe, moderate or mild.
These things are called asthma triggers. There are many kinds of triggers. Given this range, you may find it hard to figure out what starts your asthma attacks. But this is generally not true. Something usually triggers an attack.