No vaccination is required for São Paulo, unless you are planning to travel to central-western (Mato Grosso) or northern (Amazon) regions of Brazil afterwards, for which you should take a shot against yellow fever, and carry anti-malaria medication (quinine). If you're arriving from Peru, Colombia or Bolivia, the vaccination of yellow fever is required (i.e. you cannot leave these countries without your vaccination card if you're heading to Brazil). Some countries, such as Australia and South Africa, will require evidence of yellow fever vaccination before allowing you to enter their countries if you have been in any part of Brazil within the previous week. Check the requirements of any country you will travel to from Brazil.
Tap water in São Paulo is generally safe, at least when straight from the water supply system. However, several buildings can be lacking in the periodic cleaning of their cisterns and water tanks (the locals themselves tend do avoid tap water and drink bottled or filtered water instead).
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São Paulo: Health is based on work by Edson Hiroshi Aoki, Peter Fitzgerald, Marius Mollersen, Claus Hansen, Drew Lietzow, Zorn, iGuide, and others. São Paulo Travel Guide & Sao Paulo Interactive Map is available under Creative Commons Attribution-ShareAlike 1.0. See background image credits.