There are no health risks associated with travel to Spain, and no vaccination certificates are required for entry so far. With the current Covid pandemic, this could change.
In 2009, there were approximately a thousand reported cases of the H1N1 flu, also known as the swine flu, but only a handful of people died from the infection. The vaccine has been made available to the general public, with pregnant women and young children receiving priority. Additionally, the bird flu was first detected in a dead bird in July 2006, and although there is little risk to travelers, close contact with live birds should be avoided and all poultry products well cooked as a precaution. No human deaths or infections have been reported.
Spain has a reciprocal health agreement with most EU countries, providing emergency health care on the same terms as Spanish nationals. EU travelers should carry a European Health Insurance Card (EHIC). Note that the scheme gives no entitlement to medical repatriation costs, nor does it cover ongoing illnesses of a non-urgent nature, so get residency/comprehensive health insurance. As of 31 December 2020, the United Kingdom has left the EU.
Sexually transmitted diseases are common but poorly documented in Spain. Condoms are available in almost all public restrooms of major restaurants, bars, and shopping centers.