If people do not have a steady supply of good-quality water that is sufficient to meet their daily needs, this can lead to disease through two principal transmission routes.

Waterborne disease transmission occurs by drinking contaminated water.  Often, faeces are the source of contamination and the diseases are typically diarrhoea, typhoid, etc.

Water-washed disease occurs when there is insufficient water for washing and personal hygiene.

The lack of water means people can't keep their hands, bodies and domestic environments clean: skin and eye infections then spread easily.

Sanitation facilities interrupt the transmission of faecal-oral disease at its most important source, by preventing human faeces from contaminating sources of drinking-water. 

Studies suggest that sanitation is at least as effective in preventing disease as improved water supply (see definition of "improved").