Growing lice meant feeding them blood, the more human the better. At first he tested his method on Guinea pigs, but around 1933 he began large-scale testing on humans, feeding the lice human blood by letting them suck human legs through a screen. This could cause typhus during the latter phase, when the lice were infected. He alleviated this problem by vaccinating the human "injectees", which successfully protected them from death (though some did develop the disease). Weigl himself developed the disease, but recovered.