What are the most common diseases that veterinarians treat?If you know about three, it be really great :)

I think the answer to your question depends on the animal
that the veterinarian is treating. So, since the animals most commonly seen by
veterinarians are dogs, cats, birds, and reptiles, I'll give you a list of some of
the diseases most commonly suffered by these
animals.

Dog diseases:
According to discovery.com, the most commonly diagnosed conditions of both dogs and cats
are weight issues, dental problems, and fleas. Dogster.com lists the 5 most common
canine physical problems as ear infections, skin allergies, hot spots (pyoderma),
gastritis, and enteritis. Dogs can also suffer from urinary tract infections and other
conditions. Dogs should also be treated for parvovirus and other
parasites.

Cat Diseases:
Before adopting a cat, you should have it tested for two deadly diseases:
FIV (the feline version of HIV) and feline leukemia. There is no cure for either
disease, and they are highly communicable to other cats. Other diseases commonly
suffered by cats are urinary tract infections, diabetes, and upper respiratory diseases.
Cats can also develop mammory gland cancer, lung cancer, and pancreatic
cancer.

Bird Diseases: I'm not
as familiar with birds as I am with cats and dogs, so I have to rely on the source for
information. According to Alyson Burgess, the 5 most common illnesses encountered in pet
birds are Proventricular Dilatation Disease (PPD), also called wasting disease;
Psittacosis, or "Parrot Fever"; Psittacine Beak and Feather Disease (PBFD);
Polyomavirus; and Candida. I've included a link to Burgess's article so that you can
read about these
conditions.

Reptiles: Reptiles are
most commonly treated for bacterial and fungal infections. See exoticpetvet.net/reptile
for more information.

I hope this helps
you.