2.    Hosts and reservoirs. Reservoirs are animals (often birds or rodents) that carry the virus without being ill, while the hosts fall ill when they are contaminated by the virus. The West Nile Virus is a good example of the difference between a host and a reservoir. In the majority of cases, birds can live with this virus without developing any symptoms. They are reservoirs. However, the virus can cause a serious, sometimes fatal, neurological disease in humans or horses.  Humans are the host for the West Nile Virus as is the case for dengue fever or chikungunya. With the growth in travel and migration, it is clear that the number of people contaminated by one of these viruses will have a tendency to increase in countries that have hereto been free of them, such as Belgium.

3.    A vector. Haematophagous insects (which feed off blood) often play this role. By biting and sucking the blood of an infected host, the insect absorbs a certain quantity of the virus circulating in the blood. The majority of species of mosquitoes are nothing more than flying syringes. This means that the virus isn’t able to reproduce and develop inside the insect (because of some physical and immune barriers of these species). However, some vector species aren’t content with being simple flying syringes. In this case, the insect’s body serves as a “biological reactor” allowing the viral charge to be multiplied. The tiger mosquito is an excellent vector of dengue fever and Chikungunya.

4.    An environment favourable to this vector ecology, especially humidity and heat. Malaria gradually disappeared in our regions as authorities dried up the marshes whose humid terrain attracted the proliferation of certain mosquitoes.  Some vectors and some parasites cannot survive or proliferate if the temperature is too low. Aptly-named tropical diseases develop above all... in the Tropics.