"-itis" refers to an infammation. For example, tonsilitis is an inflammation of the tonsils.
"-ism" could have one of several meanings. In your example, rheumatism, the suffix refers to a disorder (e.g. autism). However, the suffix "-ism" could also refer to a movement/ideal (e.g. feminism, Nazism), or a form of prejudice (e.g. sexism). Note that these are only two possible meanings. It would be wise to consult a dictionary for a more comprehensive list.