Purebred dogs have never had a reputation for perfect health, but these breeds have, by far, become the least healthy over the past 100 years of breeding. Dr. Caen Elegans has compared photographs of dog breeds from 1915 to those of today, highlighting the health problems that have arisen due to poor breeding.
Unlike their earlier generations, the Bull Terrier has now been bred to have a thicker skull, wider abdomen, and a handful of health problems and quirks. They're known to have extra teeth and compulsive behaviors such as chasing their tails.
Boxers' noses have been bred to become shorter and more upturned which means that they have developed the problems most common for bracecyphalic (smoosh-faced) dogs. They have trouble regulating body heat as well as breathing normally. Boxers also have a high risk of cancer.
Over the years the Basset Hound has been bred to have shorter legs (so lesser mobility) as well as droopier eyes, ears, and skin (which causes infections and other issues). If you look closely at the picture you'll see that the rear legs have changed in structure, which has caused more problems.
With huge proportions, a smooshed face, and wide hips, the English Bulldog has, according to Elegans, "come to symbolize all that is wrong with the dog fancy and not without good reason; they suffer from almost every possible disease." According to a survey in 2004, the median age of a bulldog is about six years old. That's because they're so very unhealthy, and can barely mate without human medical involvement.
As you can see in the photos, the Dachshund's proportions have become more pronounced as time has gone on. Due to their long backs and short legs, they are at the highest risk of intervertebral disc disease which can lead to paralysis. They face other problems, too, especially with their limbs.
According to Dr. Elegans, the German Shepherd has declined in abilities and health due to its increase in size and change in proportions. "There was a time when the GSD could clear a 2.5 meter (8.5 ft) wall; that time is long gone."
Over time the Pug's smooshed face has become even more extreme, and with that comes all of the health problems commonly associated with that. They also have high blood pressure and heart problems. Oh, and that double-curled tail that's so desirable in pugs? That's actually a defect that can cause paralysis.
St. Bernards have been bred over the past 100 years to be oversized and have smooshed faces. This leads to a lot of diseases commonly associated with these traits. They also have excess skin, which leads to infection. In addition, St. Bernards suffer from "entropion, ectropion, Stockard’s paralysis, hemophilia, osteosarcoma, aphakia, fibrinogen deficiency." That's quite a list of health problems.