(CNN) -- The disease that led to actor-director Harold Ramis' death is one of an often-mysterious family of maladies that can starve organs and cause painful tissue damage.
Vasculitis develops when the body's immune system turns on its network of veins and arteries. Blood vessels become inflamed, restricting the flow of blood or cutting it off entirely, according to the National Institutes of Health. The disease can also cause an aneurysm, potentially leading to dangerous internal bleeding.
"Basically, the arteries can be leaking, they can be blocked or broken, and that all causes problems," said Dr. Peter Merkel, a rheumatologist and director of the Penn Vasculitis Center at the University of Pennsylvania medical school. "If you interrupt the blood supply, whatever organ or tissue is being supplied downstream is unhappy."
Ramis -- the director of "Caddyshack," "National Lampoon's Vacation" and "Groundhog Day," co-writer of "National Lampoon's Animal House" and co-star of "Stripes" and "Ghostbusters" -- died Monday at 69, four years after contracting the condition.
His agents identified the cause of death as complications from autoimmune inflammatory vasculitis, but that's "not a term that physicians would routinely use" and doesn't identify which of the more than 15 identified variants of vasculitis that Ramis had, Merkel told CNN.
None of those conditions individually affects more than 200,000 people in the United States, Merkel said -- "but if you add them all up together, it's not rare, and chances are everybody knows somebody, directly or indirectly, that is affected."
"Some of the manifestations are painful or very debilitating," he said. In Ramis' case, he suffered an infection that led to complications and forced him to relearn to walk, the Chicago Tribune reported.
Since its symptoms mimic those of other diseases, it can often be missed. The causes are largely unknown.
"We have some hints for most types of vasculitis," Merkel said. "Drugs or medications can cause it. Some infections can lead to vasculitis, such as hepatitis B or hepatitis C. But for the majority of patients of vasculitis, we don't know the cause. We're doing a lot of research to try to find it out."
Treatments usually involve steroids and "chemotherapy-like" drugs, Merkel said. Vasculitis is the subject of a great deal of federal and privately backed research, and treatments "are much better than they were," he added.
"We are making a lot of progress in treating this disease," he said.
Among those supporting those studies is the Vasculitis Foundation, based in Kansas City, Missouri. The organization said Monday it was "shocked and deeply saddened" by Ramis' death.