Hepatitis A and B
Starting with careful clinical and epidemiological studies, they discovered that virtually the entire patient population and staff became infected shortly after arrival at the institution unless they had been previously infected. Most infections had gone unrecognized until Krugman and Ward arrived. And more surprisingly two clinically distinct types of hepatitis, now known as hepatitis A and B, actually co-existed at Willowbrook. The many cases of so-called "relapse" had actually been reinfections with a different virus.