As with most diseases and illnesses, is the most important. Therefore when arriving on altitude, try keeping the altitude in mind from the moment you arrive, making sure you do not overexert yourself the first day(s), drink plenty of water and eat light food. With these tips, most travelers will only have minor discomforts when arriving to altitude.
In the jungle (and also in a small amount in Machu Picchu) it is important to protect yourself from mosquitoes. Use good repellent (preferably with DEET) throughout the day and wear long sleeve shirts and trousers in the early morning or late afternoon.Traveler’s stomach illnesses can also be an issue in some cases.
Peru has an amazing kitchen but as cooking processes and ingredients vary from what you are used at home you may want to refrain from street food and restaurants that give the impression of not being the most hygienic. Most tourist restaurants are more than find and for some tips see the restaurant list on this page.
It is also best to avoid tap water and to use bottled water for drinking and brushing your teeth.
Being a country with many days of sun throughout the year as well as altitudes make that protection from the sun is very important. In the highlands due to the clouds and winds one may not feel the sun as strong but the radiation in these parts of Peru is often more than double than on sea-level.
Therefore it is important to use plenty of sunscreen with a good factor throughout your trip. This is also true for dry season trips to the jungle as well as the summer time on the coast.