According to the Dr. Ricardo Jorge National Health Institute (INSA), which is charged with diagnosing Zika cases, the five detected in Portugal were in individuals who contracted the virus in Brazil.

The Zika virus is transmitted to humans by infected mosquitos who bite them, and cannot be transmitted from person to person.

According to the Directorate-General of Health (DGS), "the symptoms and clinical signs of the disease are as a rule slight: fever, skin rashes, joint pain, conjunctivitis, and head- and muscle aches", but "there are suspicions ... that the disease may trigger foetal alterations during pregnancy, in particular microcephaly."

Cases of the virus have been found in Brazil, Cape Verde, Colombia, El Salvador, Fiji, Guatemala, Mexico, New Caledonia, Panama, Paraguay, Puerto Rico, Samoa, Solomon Islands, Surinam, Vanuatu, Venezuela, Martinique, French Guiana and Honduras.

On Monday the World Health Organisation warned that Zika will continue to spread through the Americas.