Measles outbreaks demonstrate how easily unvaccinated travellers can cause a disease to reappear in countries that had previously eliminated or interrupted endemic transmission. As the World Health Organization says: “The measles virus can spread wherever immunity gaps exist.”13

For some diseases, however, outbreaks occur when a vector (such as a biting insect) which is initially disease-free becomes established in an area. Local transmission can then occur when it feeds on a disease ‘reservoir’, such as an infected human travelling in the area. This one reason for the transmission of ‘tropical’ diseases now occurring in southern Europe.14

Epidemics of the viral disease dengue fever disappeared from Europe in the mid-Twentieth Century when the mosquito vector Aedes aegypti died out in the region. However, in the past couple of decades, the Aedes albopictus mosquito has arrived and become established in southern Europe.

In 2013, a WHO European Region report warned that the “invasive” Aedes albopictus had transmitted dengue fever and chikungunya locally and had the potential to transmit yellow fever virus. As the species has now been found to be a vector for Zika virus in Africa, there is a fear it could eventually transmit the disease in southern Europe.14,15