The bubonic plague. Scarlet fever. Polio. Diseases of the past, and no longer on this Earth, right?
Many old-time diseases are still present today, but fortunately many of them aren't as prevalent or dangerous as their ancestral versions. But why are these diseases still around?
For infectious diseases, a big part of it has to do with the organism in which the diseases are carried -- it's a lot easier to eradicate a disease if it's only spread from human to human, than if it's spread from animals to humans, said infectious disease expert Dr. Geoffrey A. Weinberg, a professor of pediatrics in the Division of Pediatric Infectious Diseases at the University of Rochester Medical Center.
So far, there have only been two infectious diseases that have been completely eradicated from the planet: smallpox, and the animal disease rinderpest, he said.
"Those were easier to eradicate from the world because they were single strains of organisms -- both were viruses -- and they only infected one type of creature," Weinberg told HuffPost. "Smallpox only infected humans -- it was easy to recognize, and there was only one strain of it -- so a vaccine was easily producible. All those things combine to make it something that if people get their minds together and can recognize a disease, they can eliminate it."
With illnesses like the flu, on the other hand, the virus that causes it mutates a lot and can live in a number of different organisms, including humans, pigs and birds, he said.
Take bubonic plague, for example. The disease is most commonly transmitted to humans through flea bites, and the fleas live on rodents. It's hard to completely extinguish bubonic plague in rodents and fleas, since there are so many of them around the world, Weinberg said. The same goes for spore diseases like anthrax, where it's in the soil -- "you can't get rid of that unless you concrete the entire world, because it's in the soil."
"Basically, the factors that lead to why diseases stick around, or come back, is because sometimes the germs change a little," Weinberg said. "Sometimes they're germs you can't get rid of, thats why we have to keep it suppressed by protecting people with a vaccine. Other germs, they're common and we don't know how to get rid of them. And then some germs, like influenza, are changing their coats. You're always chasing after them."
Take a look at five old-time diseases that still persist today.