Diseases can be a major part of any world because they can do a lot of damage to a population of people. Epidemics such as the black plague helped change the flow of history and influenced certain events. It is possible that even if you don’t show any people infected with diseases directly in your story that they are at least a major part of your world’s history.
There is a good chance that diseases or any other illness don’t ever show up in your stories. If they do it is a good idea for you to research diseases especially if you want to use a real disease in your story. It is a good thing to remember that symptoms are caused by either the body trying to fight off infection such as with a high fever, or directly caused by the disease fighting against the body.
You may choose to make up your own disease. Just try to make the symptoms make sense. You also may want to decide how the people in your world view diseases and people infected with them. Some diseases such as leprosy have historically caused a lot of discrimination because people fear catching such diseases even if there is next to no chance of them doing actually catching anything by interacting with those infected.