When you think about the things that you can be fired for, you probably think one that could result in your termination would be taking off work for an illness. In some cases, illnesses are actually protected; for instance, if a woman gives birth, she can't be fired for needing to go to the hospital or for having to take time off. If you need emergency surgery and miss a shift, you shouldn't be fired for that.
However, there are cases when a company can fire you for your illness under the guise of another reason. For instance, if you have always had good corporate reviews in the past and now find that they're all negative (and coincide with your illness), then you may be seeing some discrimination. How can you prove that, though? Unless the business comes out and says it's firing you because of your illness, it can be hard to prove.
There are reasons why a company would want to let you go if you were ill often. For instance, hiring a temp to take your place can be exceptionally expensive and cost the company thousands from its budget. If you can't return to the job in the same capacity, then the employer may not want to continue to have you as an employee, because someone else may have to pick up the slack. Insurance costs also rise when a catastrophic illness requires ongoing care.
Fortunately, if you find that you are being terminated because of your illness or can link several actions or behaviors to your illness and its effects, you may be able to make a claim in court.
Source: The Huffington Post, "Fired Because of Illness," Erin Havel, June 08, 2016