25 years, 36,000 postcards.

Medicine Araveeti Ramayogaiah was an Indian paediatrician who years before the internet was passing the word about preventative medicine, especially in relation to disease, to poor areas by sending out postcards to the vulnerable and sick who couldn't afford private health care:

For almost 25 years, Ramayogaiah wrote and sent postcards to India’s poor, especially women, telling them about ways to prevent—rather than cure—diseases. The good doctor died in Hyderabad in September this year at the age of 65. 

The inexpensive postcard was Ramayogaiah’s solution to private hospitals, which are typically inaccessible and unaffordable for many of India’s poor. 

In all, he wrote around 36,000 postcards to patients, acquaintances and strangers—explaining basic habits like boiling water and washing hands, and how to prevent commonplace ailments like diarrhea.