Water, and Sanitation. No mattter how primative the level of technology or infrastructure, the need to get potable water in and non-potable waste out is a universal need for all human settlements.

Water is cheap, and soap and emergency toilets aren't that expensive.
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Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane