You have thousands of different cell types in your body - each type is unique, and each has an important part to play in keeping you healthy. Some of these cell types play an important role after they've stopped working. For example, the cells in your outermost layer of skin are really only there for protection. None of the components inside of these cells work, but they are necessary boundaries between the more sensitive layers of cells beneath and the outside world.

However, most cells have important roles to play while they are still "living" and functional. You have cells in your intestine that need to absorb the nutrients from the food you eat. There are muscle cells in your heart that need to respond to other cells that tell them when to contract and relax, keeping your blood pumping. You have billions of cells in your brain that process the things you experience in the world (touch, sight, taste, sounds, and smells), make thoughts and memories, and tell other parts of your body what to do.

Some diseases impact one cell type in particular, and so the symptoms a person would get are a result of that cell type not working correctly. For example, a lung disease would make it difficult for a person to breathe, which in turn would deprive the other cells of the body of oxygen and make it harder for those cells to get rid of CO₂.