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.

50 to 80 percent of adults already carry symptom-free herpes-based cytomegaloviruses, so scientists want to engineer them to include fragments of more potent viruses, allowing for a transmissible vaccine without the risk of mutating back into something more nasty.

Think outside the box: Beyond preventative vaccines, scientists are also designing a transmissible treatment for HIV called therapeutic interfering particles (TIP). These TIPs steal the same proteins used by HIV to replicate, in effect taking away the virus' resources - and they can be transmitted from person to person.