Roughly 5% of people who have asthma are unable to bring it under control and have severe asthma. Their bodies, for whatever reason just do not seem to respond nearly as well to the regular treatments given. Consequently, this person would be described as therapy-resistant. Since their bodies do not respond to treatment, they much more commonly express symptoms of asthma, and these are normally quite severe. An example of this is struggling to breathe and general breathlessness most, if not all of the time. They are also much more prone to more serious asthma attacks, which will require immediate medical attention and probably hospital admission.
One of the main causes of severe asthma is the presence of other underlying medical conditions. Remember that everyone who has asthma has good and bad patches, so just because someone is exhibiting symptoms at one point does not mean their asthma is classed as severe; rather they may be simply going through a bad patch. To diagnose someone as having a case of severe asthma, they must undergo a varied testing program by a multidisciplinary team in a specialist asthma clinic. This includes checking for underlying health conditions and testing how their body responds to a diverse range of medicines.