Are there drugs to treat autoimmune diseases?

Yes. Yes. There are many medications to treat autoimmune diseases, depending on the specific disease.
Yes. Many autoimmune diseases can be treated with medicines that reduce autoimmunity -- many of which may also reduce the immune system's infection-fighting ability by varying degrees. Some autoimmune diseases are treated in other ways, eg, replacing the hormone from a damaged gland (thyroid -- with pills; or pancreas-- with insulin). Symptoms can be treated as well, with pills or alternative therapies.