C'mon it's a trick question. The answer is vaccination.
Nah. April Fool's!
The UN says driving. Their top 10 causes of death are [my comments]:
1. Ischemic heart disease
2. Cerebrovascular heart disease (such as stroke)
3. Lower respiratory infections (pneumonia)
4. HIV-AIDS
5. Chronic obstructive pulmonary disease
6. Perinatal conditions [that's pretty broad, isn't it?]
7. Diarrheal diseases [cholera! dysentery!]
8. Tuberculosis
9. Trachea, bronchus and lung cancers
10. Road traffic injuries
11. Diabetes
12. Malaria
Uh-huh. And what would worry most Americans? Probably being struck by lightening or flying or something.
Read an op-ed at An "Epidemic on Wheels," By Norman Y. Mineta, Washington Post Monday, March 31, 2008; Page A19