Помню памятки-брошюры то ли от воз, то ли от cdc, где подчеркивалось, что мытье рук значительно уменьшает процент желудочно-кишечных и легочных заболеваний.
Гельминтозов так уж точно. Простите. 
Вот из CDC (федеральное агентство по контролю заболеваний США) кусок:
Teaching people about handwashing helps them and their communities stay healthy. Handwashing education in the community:
Reduces the number of people who get sick with diarrhea
by 31% 1, 2
Reduces diarrheal illness in people with weakened immune systems
by 58% 3
Reduces respiratory illnesses, like colds, in the general population
by 16-21% 2, 4
Цифры в конце каждой строчки - ссылки на работы:
References
Ejemot RI, Ehiri JE, Meremikwu MM, Critchley JA. Hand washing for preventing diarrhoea. Cochrane Database Syst Rev. 2008;1:CD004265.
Aiello AE, Coulborn RM, Perez V, Larson EL. Effect of hand hygiene on infectious disease risk in the community setting: a meta-analysis. Am J Public Health. 2008;98(
:1372-81.
Huang DB, Zhou J. Effect of intensive handwashing in the prevention of diarrhoeal illness among patients with AIDS: a randomized controlled study. J Med Microbiol. 2007;56(5):659-63.
Rabie T and Curtis V. Handwashing and risk of respiratory infections: a quantitative systematic review.Trop Med Int Health. 2006 Mar;11(3):258-67.
