Viral diseases are a moving world with epidemic upsurges, the disappearance of some infections and the temporary or definitive appearance of others. Each year our country is stricken by flu epidemics, but at some points of time unexpected new flu viruses may cause a worldwide pandemic. Presently a number of viral diseases have disappeared or are disappearing from our country, as smallpox, poliomyelitis, rabies, measles or rubella. Many factors, some changing others more persistent, may explain the high appearance rate of new viral infections: increasing intercontinental travel, globalization of transportation, increased contacts of man with new ecological niches, great variability and adaptability of viruses, climate changes, increasing density of populations and urbanisation, changing human behaviour (more sexual freedom, use of intravenous drugs, etc...), changing social and economic conditions, which influence housing, nutrition, access to water, etc...). Viruses take all opportunities in our changing world and have many more surprises in store for us.