Utilities may be offline for an extended period of time following a disaster. Underground water pipes or septic systems may be broken or leaking, leading to contamination. Assume all water not sealed in containers (including well water) following a large disaster is contaminated. Boil water for at least 10 minutes or add 8 drops of pure liquid bleach per gallon to make water safe to drink (avoid cloudy, dirty water). After adding bleach, let water sit for 30 minutes before drinking.
Food can easily become contaminated as well. If the power is out, previously frozen and refrigerated food that has been at room temperature must be disposed of. If exposed to nuclear, biological, or chemical attack, all fresh, frozen, plastic and paper packed foods need to be disposed of. Canned food is generally the safest, unless the can appears punctured, bloated, or rusted. Be sure to wash cans with purified water before opening them to be sure that contaminants do not get into the food.
Any bottled goods that have been submerged in water should be disposed of, as well as any dry foodstuffs that have become wet.
Wash your hands frequently and thoroughly, since this is the main vector with which people fall ill. Be sure to use sanitary practices when preparing food, and never use charcoal/gas barbecues indoors. The carbon monoxide gases they give off can quickly kill.
Finally, take care of yourself by eating regularly, trying to sleep on a normal schedule, and taking normal medications. Much of the stress following a disaster situation can be alleviated by doing these simple things.