Vitamin deficiencies, in addition to outright starvation and hygiene (cleanliness) issues were the biggest killers of the past- and of the poorest people now.  They don't kill first world people often now for four reasons:

1) Clean water to drink. 
2) Nutritionally required vitamins.
3) Calories.
4) Protein and essential amino acids.

4 things. Lack of 4 Simple things killed the majority of people through time. 
Cheap in first world countries today, and easily stocked up on. 
Of course having a plan for fulfilling those needs when the stockpile runs out is also important.

Humanure composting returns much of the essential trace substances to the land- I doubt anyone wants to eat anything grown directly in humanure, but growing a green composting crop from that to feed your animals or compost for the garden is not unreasonable. 

A diet of homegrown vegetables from your own garden, locally sourced protein, sufficient well filtered water, and a few multi-vitamin supplements (cycled back into the local area bio-sphere with Humanure and composting) to fill out any lack will go far in improving your life and the lives of those who come after us.  Metabolic Iodine, Vitamin C,  Salt, are all usually in short supply for inland and northern areas. Other deficiencies of a mineral sort may exist in your local area and can be dealt with, by sufficient one time inputs or stockpiles and retention/ recycling methods.
Have your area checked for what it is short on and move to deal with the issue.   

Keeping clean will become harder as outside chemical sources get more expensive, local sources and reduced usage should allow anyone with practical sense to prepare for that in the future.