Arthritis an informal way referring to joint pain (also referred to as arthralgia). There are over 100 different types of arthritis. When four or more joints are involved, the arthritis is referred to as polyarthritis. When two or three joints are involved, it is referred to as oligoarthritis. When a single joint is involved, it is referred to as monoarthritis. Arthritis is one of the leading causes of disability in America. More than 50 million adults and 300,000 children have some type of arthritis.