Buy travel insurance. Although hospitals by law must treat you even without insurance, and they may have a hard time tracking you down to pay the whopping bill, you shouldn’t risk being denied care as an uninsured traveler. The United States has excellent specialists but a simple checkup with a physician costs over $100. Medicine costs are outrageous to the point that many Americans cross the borders to Mexico and Canada to buy their pills. The traveler here should bring pain killers and antibiotics from home in properly labeled bottles for customs.
America is a safe country except in the seedier parts of cities like Washington DC, Detroit and Los Angeles. Shootings may be frequent in these cities but rarely carry over to places of interest. Don’t fuck with Americans though, they do all own guns. Americans keep things locked and theft from valuables left in cars is the only really common crime (besides downloading music.)
Obesity is the number one health worry and you will see why from the first time you order a meal in any restaurant. Portions are huge, sharing is strongly discouraged by wait staff, and too many people never exercise.
Tap water is safe to drink although bottled water is popular thanks to the persuasive powers of advertizing. America is a clean country unless you happen to be Japanese.
Hikers going on multi-day treks should log in at ranger stations, it’s not hard to get lost in the huge wilderness. Mosquitoes and poison oak/ivy are a nuisance.