A health crisis similar to the Black Death could happen, as seen with ebola and the Zika virus. The Black Plague was spread through so thoroughly through highly populated trade routes after the rats brought plague infected fleas on ships. These days, we have even more means of efficient travel including flights across the world, which allows an easy spread of disease across large areas. There are two likely ways such a widespread disease could occur: through unvaccinated kids, or spreading from areas with little sanitation. The ¨anti-vax¨ movement could easily create a problem in that unvaccinated children will catch preventable diseases, such as polio or the measles, and spread them to younger kids unable to get vaccinations, or to children who also don´t take their vaccines. Also, areas with little sanitation are breeding grounds for diseases, and if anyone leaves that area, say through a flight, they can spread it to a different continent and start an epidemic.
