news @ nature.com - Potatoes pack a punch against hepatitis B - Plant that contains vaccine shows promise in human trials.:
Unlike travellers' diarrhoea and the Norwalk virus, the hepatitis B virus did not evolve to survive in the gut, which makes the success of this edible vaccine all the more surprising.
After producing potatoes that contain vaccines against E. coli and Norwalk virus, they turned their attention to hepatitis B. But the antigens to hepatitis don't occur in the gut, so it took special effort to get those antigens to move into the bloodstream. Evolution put up roadblocks, science went around.