to spread through air, mosquitoes or unclean water, particularly in urban areas. T The globe's growing population creates greater opportunity for diseaseto spread through air, mosquitoes or unclean water, particularly in urban areas. T he United Nations predicts that 66% of the global population will live in urban areas by 2050. Shutterstock

As humans expand into previously uninhabited territories such as forests, they are more likely to come into more frequent contact with wild animals and, inevitably, new infections. Shutterstock

Evidence suggests climate change is causing greater numbers of heat waves and flooding events, bringing more opportunity for waterborne diseases such as cholera and for diseases carried by mosquitoes. Shutterstock

According to experts, infectious agents can live in humans during their incubation period -- the time between infection and the onset of symptoms -- meaning that travelers can transmit an infection to another region even though they don't appear to be sick. International tourist arrivals reached a record of almost 1.2 billion in 2015. Shutterstock

If a country is on the brink of breakdown from civil unrest, its ability to handle an intense and sudden outbreak could bring its people to their knees -- and allow the infection to flourish. Shutterstock

In the information age, new means of communication bring higher levels of fear and multiple ways to spread it, experts believe. More than 90% of the world's population will be covered by mobile broadband networks by 2021, according to the UN. Shutterstock