Animals, somewhat inevitably, get ill. (As a minor aside, this is one of the great fallacies about life in the wild that animals live in some kind of paradise where everything lives free and happy and never gets eaten alive by a predator, or starves to death, or gets and infected wound and disintegrates from gangrene for example. Anyway, while of course most illnesses only affect soft tissues, some diseases and infections will leave a mark on the bones themselves and obviously as a palaeontologist these do turn up in the fossil record.
There are all kinds out there (I’ve mentioned one on a Mamenchisaurus before) but of course it is useful to see these things in modern animals to see what effects cause which pathologies and then try to track them on the fossils. There are fossils (including dinosaurs) diagnosed with arthritis, cancers and pathologies resulting from infected injuries. In this case, this is the jaw of a hippo which has clearly had something go quite wrong (though what I’m not sure, though my guess is a nasty infection). The bone is swollen and pitted at the base of the tooth is actually visible – this is clearly not natural and looks very different to the other side of the jaw. These kinds of thing at least can give you an idea if the feature you are looking at is a weird bit of anatomy or the result of a pathology – it’s not always easy to tell.