Mengapa orang tidak membuat jembatan?

Sumber: Cueto, M. (2004). The origins of primary health care and selective primary health care. American journal of public health, 94(11), 1864-1874.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448553/pdf/0941864.pdf

http://cdrwww.who.int/global_health_histories/seminars/presentation02.pdf