You won’t require any vaccinations to visit Portugal unless you are coming from an infected area. The only exception being yellow fever, a vaccination for which is required when travelling from an infected area or when travelling to the Azores or Madeira. Make a point of checking with your tour operator or local Portuguese Embassy if in any doubt.
Some vaccinations are recommended nonetheless. The CDC and WHO recommend the following: hepatitis A, hepatitis B, rabies, meningitis, polio, measles, mumps and rubella (MMR), Tdap (tetanus, diphtheria and pertussis), chickenpox, shingles, pneumonia and influenza.
It’s very important to note that in 2012, a dengue fever outbreak was reported in Madeira with 18 confirmed cases. To date, however, the mosquito carrying the fever is not present in mainland Portugal which lies 1000 km from Madeira. There is no vaccine for dengue fever, the only means of prevention is to avoid mosquito bites. More information on dengue fever.
Although Portugal is an industrialized country, bug bites can still spread diseases. Try to avoid bug bites while spending time outside or in wooded areas: cover exposed skin by wearing long-sleeved shirts, long pants, and hats, and use an appropriate insect repellent.
It can get very hot in Portugal, especially in the summer, so don’t forget to use sunscreen. Swim only in designated swimming areas and obey lifeguards and warning flags on beaches. And don’t dive into shallow water!