Visitors should drink only bottled water, which is widely available. Do not drink tap water, even in major hotels. Agua con gas is carbonated; agua sin gas is plain.
You are safer eating fruits you can peel.
Avoid eating from street vendors.
If you are thinking of visiting the jungle, remember that a yellow fever vaccination is mandatory.
:: ALTITUDE SICKNESS
If you are planning to visit cities 8200 feet above sea level, like Cuzco (11,000 ft) or Lake Titicaca (13,000 ft), shortness of breath and heart pounding are normal, given the scarcity of oxygen. Some people may experience headache, loss of appetite, extreme fatigue and nausea. Most symptoms develop the first day at high altitude, though, occasionally, travelers have delayed reactions. The best advice is to rest on your first day in the highlands. Drink plenty of liquids, including the local remedy: mate de coca or coca - leaf tea (It’s perfectly legal). Avoid alcohol and heavy food intake.
You can avoid altitude sickness taking 500 mg a day of acetazolamide (Diamox) taken 24 hours before departure and continued up to 48 hours after arrival to these cities. Acetazolamide should not be given to persons allergic to sulfa drugs.