Asthma can be caused from four primary sources, namely allergens, irritants, weather and infections. Allergens include pollen, dust, animal hair, mold etc. Irritants could be in the form of air pollution and smoke. Extreme weather and change in weather patterns can also contribute to developing asthma. And lastly, persisting infections like cold and flu can also lead to asthma if the cold is not treated for a long time.