Asthma is a complex condition. It is difficult to say for sure what causes it but, so far, we suspect that genes play a part in predisposing someone to developing the condition. We also know that there isn't a single asthma gene. Instead, the combined effects of several genes probably work together to produce a susceptibility to asthma; in the same way, other genes lower the chances of developing it.
As countries and communities have modernised, it could also be that changes in lifestyle (diet, housing and hygiene) have led to a higher prevalence of allergy. It may well be that communities eating a lot of natural and fresh foods, which contain lots of chemicals called antioxidants, are relatively well protected against developing allergies.
There is still so much to find out about the allergic process and what it is about our environment that has caused the phenomenal increase in allergic disease throughout the modern world. In the meantime, allergy is, unfortunately, part of everyday life for many of us, and a number of so-called 'triggers' can set it off, such as viral infections (colds or flu), allergies (to pollen, animals, housedust mites), irritants (cold air, tobacco smoke, chemical fumes) and exercise.
Although it is unlikely that you will be able to avoid all your asthma triggers all of the time, steering clear of them will help to keep your symptoms at bay. Try to keep a record of the times and situations when your asthma is worse. This will help you identify what your asthma triggers are.