Viruses have become world travelers, thanks mainly to the growing mobility of humankind – and of insects. Climate change is also a source of the exotic infections currently spreading in Europe.

According to the Robert Koch Institute (RKI), the German government institution responsible for disease control and prevention, we will increasingly have to deal with new, little-known viruses.

Americans got an inkling of what that feels like in 1999, when dead crows started falling out of trees in New York’s Central Park. The virus struck first birds, then human beings, sometimes fatally. The cause? A mosquito that had hopped a plane from Israel to New York, carrying the West Nile virus.

That virus was originally discovered in a Ugandan woman in 1937. It is now widespread in the U.S., where it has caused particular damage this year. According to the American Centers for Disease Control and Prevention (CDC), in 2012 more than 3,100 people were infected, 1,600 of whom developed the dreaded neurological symptoms. Over 130 people died from the disease this year in the U.S. alone.

The present outbreak of West Nile Virus in the U.S. makes one thing clear. People in Western countries need to realize that exotic infections they never thought of worrying about are something to be aware of. This does not apply only to the United States. Many people in Austria, Italy, Spain and a number of Eastern European countries have contracted West Nile Virus, although most of them did not get the disease at home but brought it back from their travels. So far there have been  only a few cases in Germany, but experts believe that it is only a matter of time before the virus spreads there as well.

Along with West Nile Virus, there are a number of other viruses that periodically travel from the tropics to Europe and the U.S. For many European tourists, the dengue virus is an unwelcome souvenir brought back from a trip to Asia, Africa or South America. The RKI has recorded over 300 people suffering from dengue fever this year in Germany. A man in Greece, who apparently became infected in Greece itself, died of the fever in early September, the first such case in Greece in over 80 years.

Even more exotic are the Crimean-Congo hemorrhagic fever and the Chikungunya virus, mostly contracted in Africa or Asia. Many Europeans learned about these viruses for the first time in 2007, when about 200 Italians fell ill after a traveler to India brought the Chikungunya virus back to the province of Ravenna. Two cases have since cropped up in France, both patients infected in Europe. "When carrier mosquitoes reach Germany, it’s entirely possible that people in Germany will become infected with this virus," said Professor Klaus Stark, RKI's tropical disease expert.

Airplane cabins or baggage holds

Mosquitoes are the carriers of diseases like West Nile, dengue and Chikungunya fever, transmitting the virus from infected humans or animals whose blood they have sucked to whomever they bite next.

The crux of the problem, however, is the increasing mobility of people, and also of insects. The Asian tiger mosquito or the Japanese bush mosquito can travel to Europe from their native countries in airplane cabins or baggage holds. The surge in intercontinental travel is globalizing disease.

Mosquitoes that make the trip successfully also benefit from climate change. Temperature rises in Western countries make it possible for insects and viruses to proliferate rapidly. Sometimes mosquitoes even benefit from apparently disadvantageous conditions.

Researchers believe, for example, that the severe drought in the U.S. this summer favored the spread of the West Nile Virus because the insects congregated in the few remaining damp zones. American CDC experiments have also demonstrated the consequences of high temperatures. At a temperature of 17° Celsius (62.6 Fahrenheit) it took 30 days for an infected mosquito to pass on the West Nile Virus, whereas at 30° (86 F) it took only five days.

"High concentrations of mosquitoes in residential areas and of people infected with the viruses are prerequisites for dengue and Chikungunya," Prof. Stark says. "In the case of West Nile Virus, hosts like birds are an additional factor. We can‘t exclude the possibility of isolated cases of these diseases in Germany over the next few years." However, neither dengue nor the Chikungunya virus should be able to get a strong foothold in Germany, he adds. "Monitoring of mosquitoes, a well-functioning health system, and climactic conditions will prevent that."

There is presently no cure for dengue or West Nile fever, mainly because health problems are caused less by the viruses themselves than by the ways in which the body tries to fight them.

Pharmaceutical companies are therefore trying to develop vaccines against the diseases. There is already one for yellow fever, which is why the infection rarely hits travelers. Several companies are presently working on a vaccine against the dengue virus, but it is a particularly difficult one because it crops up in four different forms. For the vaccine to be successful, it must protect against all four forms.

The vaccine already developed by Sanofi Pasteur protects against only three forms, as a test conducted on 4,000 Thai children showed. If people become infected by the fourth variation of the fever, they could become even sicker than those who are not vaccinated at all, warns Jonas Schmidt-Chanasit of the Hamburg-based Bernhard Nocht Institute for Tropical Medicine.