On Airs Waters and Places, Part One- Hippocrates

Whoever wishes to investigate medicine properly, should proceed thus: in the first place to consider the seasons of the year, and what effects each of them produces for they are not at all alike, but differ much from themselves in regard to their changes. (1) Then the winds, the hot and the cold, especially such as are common to all countries, and then such as are peculiar to each locality. We must also consider the qualities of the waters, for as they differ from one another in taste and weight, so also do they differ much in their qualities. (2) In the same manner, when one comes into a city to which he is a stranger, he ought to consider its situation, how it lies as to the winds and the rising of the sun; for its influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. These things one ought to consider most attentively, and concerning the waters which the inhabitants use, whether they be marshy and soft, or hard, and running from elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hollow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor, and not given to excess in eating and drinking. 

Source: http://classics.mit.edu/Hippocrates/airwatpl.1.1.html

(1) In Winter, people tend to remain inside together for longer periods. During this period therefore the Influenza viruses A and B tend to afflict Americans, which is why seasonal vaccinations are advised between September and October. Treatment includes Rest, Acetaminophen and Hydration, or if the symptoms have been going less than 48 hours, Oseltamavir.

(2) Fluoride-fortified water has been shown to reduce childhood dental disease. Some of the waterborne-only human diseases in untreated water include:

Bacteria: Vibrio cholera, Leptospira (contaminated with animal urine)
Viruses: Adenovirus, Hepatitis A, Hepatitis E, Poliovirus
Parasites: Giardia lamblia, Entamoeba histolytica (in immunocompromised), Cryptosporidium, Naegleria fowleri (warm fresh water brain amoeba),
Nematodes: Enterobius vermicularis, Ascaris lumbricoides
Dracunculensis medininsis,
Trematodes (Flukes): Chlonorchis sinensis (Chinese liver flukes), Schistosoma haematobium (swimmers at risk), Schistosoma japonicum, Schistosoma mansoni