Scientists around the world are exploring new ways to spread vaccines in rural, remote areas that need them most. One way to do so? Letting nature do the work via contagious vaccines and treatments for viral diseases, per Popular Science.
How it works: Vaccines use dead and weakened viruses to train the body's immune system to attack a pathogen, so it might be possible to engineer those viral vaccines to be transmissible between humans.
One big drawback: Viruses can mutate significantly with each new generation, perhaps allowing a dead or weakened virus to become active once again.