Illness is the opposite of health. It is often difficult to draw the line between the two. If someone has chicken pox or measles or suffers from appendicitis, then they are certainly sick. A doctor examines and treats the sick person.
But it becomes more difficult even with a minor illness. If you have a cold, for example, you can still go to school or work. Only when the illness gets worse does the doctor issue a certificate saying that you can or even should stay at home.
Diseases can be distinguished from each other in various ways and grouped together. For example, it is important to distinguish between contagious and non-contagious diseases. A cold, for example, is not bad, but it is contagious. There are viruses in the spit that we blow out in small droplets when we cough. Cancer, on the other hand, can lead to death, but it is not contagious.
Childhood diseases such as mumps, rubella or scarlet fever also form a special group. Most of them are not contracted later as adults. Children can be vaccinated against many childhood diseases to prevent them from getting sick in the first place.
Hospitals have special departments for different diseases: For the heart, lungs, digestion, muscles or skin, but also for cancer, the immune system and so on. Many doctors have a practice for only one of these diseases. So they are specialists. A doctor who takes care of all diseases is a family doctor. He tries to treat the disease himself or sends you to a specialist.
Something different from the disease is the disability. A disabled person may not be able to do something. A blind person cannot see. But that does not make him ill, it is normal for him. Being blind does not go away for him at some point. However, there are chronic diseases: "chronic" is a word for "time" and means here that one has the disease for a very long time or even forever.