Simplify the following text for a Grade 13 native English-speaking person.
Original: Other forms of arthritis include Rheumatoid arthritis ( which causes the bodyâ s immune system to attack the bones ) , psoriatic arthritis , and septic arthritis ( when an area is invaded by bacteria ) .
Simplified: Other arthritis forms are rheumatoid arthritis , psoriatic arthritis , and related autoimmune diseases . Septic arthritis is caused by joint infection .