The average cough creates a cloud of more than 3,000 moisture droplets that hang in the air for hours.
And bodily fluids can harbour some nasty stuff. Take, for instance, the Black Death; it killed more than 200m people in the 14th Century alone. Or smallpox, which used to kill 400,000 Europeans every year. Smallpox was eradicated in the 20th Century, but if it somehow got back into the natural world it could create a doomsday scenario. That’s why both the US and Russian governments keep stocks of the disease in top secret labs.
But the world is already full of infectious diseases. And germ warfare – using those bugs to win battles – has been going on for a long time. Around 3,500 years ago, a race of Middle Eastern warriors known as the Hittites hatched a plan to leave six sheep at the gates of an enemy city. When the unsuspecting locals took in the livestock, ticks from the sheep gave them a dose of rabbit fever, a contagious bacterial disease that could have killed half of them and left the city for the taking.
As technology grew, so did our germ warfare ambitions. During World War II, the British government tested the deadly disease anthrax on the Scottish island of Gruinard, which then had to be quarantined for 48 years. And in the 1930s, the USSR carried out tests that left an entire island in the Aral Sea uninhabitable, packed with rabbit fever, plague, typhus and Venezuelan equine encephalitis.
Even worse, in 1971, Russia accidentally released weaponised smallpox, which infected 10 people and killed three of them before it was dealt with.
To find out more, watch the video above.