In 1981, the following information came to the attention of Barry Marshall, a gastroenterologist at the Royal Perth Hospital in Australia. Household members of ulcer patients do not develop antibodies against Helicobacter. However, clinical staff involved in obtaining samples from ulcer patients develop antibodies against Helicobacter. If acid-suppressive therapy is combined with antibiotics, ulcers usually do not recur. Marshall concluded that ulcers are an infectious disease.
1) What caused Marshall to reach his conclusion?
2) What additional proof would he needed?