Travellers who spend at least 24 hours in certain southern European countries face restrictions when donating blood in Switzerland.
The reason is the increase in cases of West Nile virus, which has been recorded in neighbouring France and Italy as well as south-eastern European countries.
So far there have been no reports of West Nile virus in Switzerland this year. If five cases are reported, the Federal Office of Public Health warned that it will require that every blood donor be tested.
Swiss health authorities have introduced a 30-day waiting period for potential donors who have just returned from a country where the virus has been reported.
The virus spreads via migratory birds and local mosquitoes with an incubation period often days, and infected people often show no symptoms.