Warfare has long been linked to disease. In fact, infectious diseases sometimes kill more soldiers than do battle wounds. Infected soldiers allow microbes to enter new ecosystems and infect civilians, sometimes with disastrous results. Soldiers may also pick up infections abroad and carry them back home.

During a war, civilian populations are equally at risk from the breakdown of infrastructure and public health systems, and the scarcity of medicine. People are often forced from their homes into crowded, unsanitary refugee camps.

If that isn't enough, warfare also destroys ecosystems. Animals carrying disease-causing microbes may thrive in such altered environments.