Nutrition and obesity
 
Human food such as ice cream can lead to ill health and obesity in dogs.

Feeding table scraps to a dog is generally not recommended, at least in excess. Dogs get ample correct nutrition from their natural, normal diet. Otherwise, just as in humans, their diet must consist of the appropriate mix of nutrients, carbohydrates, and proteins, with the appropriate mix to provide all of the minerals and vitamins that they need. A human diet is not ideal for a dog: the concept of a "balanced" diet for a facultative carnivore like a dog is not the same as in an omnivorous human. Wild and feral dogs can usually get all the nutrients needed from a diet of whole prey and raw meat. In addition, the scraps often consist of fat rather than meat protein, which in excess is no better for dogs than it is for humans. While not all human delicacies are acutely toxic to dogs, many have the same chronically unfortunate results as they do for humans. Lastly, many people overfeed their dogs by giving them table scraps and human food such as ice cream. Dogs will usually eat all the scraps and treats they are fed, which is more than often too much food.

Photo: fda.gov
 
The result of too much food is obesity, an increasingly common problem in dogs in Western countries, which can cause numerous health problems just as it does in humans, although dogs are much less susceptible to the common cardiac and arterial consequences of obesity than humans are.

Additionally, the feeding of table scraps directly from the table can lead to trained begging behavior on the part of the dog, or even encourage the dog to reach up and take food directly from the table. These are normally seen as undesirable behavioral traits in a dog.

Obesity can be a sign of other serious ailments such as Cushing’s Disease which is characterized by weight gain, appetite increase and lethargy in primarily older dogs.

A modern trend in canine diets is raw feeding of whole meats, bones and little filler material.
  

Bacterial diseases in dogs are usually not contagious from dog to dog; instead they are usually the result of wound colonization, opportunistic infections secondary to decreased resistance, or secondary to other conditions. These examples are not considered infectious diseases because they do not satisfy Koch’s postulates - for example Staphylococcus intermedius, a commonly isolated bacteria from skin infections in dogs, would not cause pyoderma when introduced to a healthy dog. In all likelihood that type of bacteria is already present on the skin of a healthy dog.

Photo: pondpoint.com

There are some bacteria that are contagious from dog to dog. The most notable of these are Bordetella bronchiseptica, one of the causes of kennel cough, Leptospira sp, which cause leptospirosis, and Brucella canis, cause of brucellosis in dogs. There are also common tick-borne bacterial diseases, including Lyme disease, ehrlichiosis, and Rocky Mountain spotted fever.

Leptospirosis

Leptospirosis is a zoonotic disease caused by bacteria of the genus Leptospira. Humans and dogs become infected through contact with water, food, or soil containing urine from infected animals. This may happen by swallowing contaminated food or water or through skin contact, especially with mucosal surfaces, such as the eyes or nose, or with broken skin. In dogs, transmission most commonly occurs by drinking puddle, pond, or ditch water contaminated by urine from infected wildlife such as squirrels or raccoons. The liver and kidney are most commonly damaged by leptospirosis. Vasculitis can occur, causing edema and potentially disseminated intravascular coagulation (DIC). Myocarditis, pericarditis, meningitis, and uveitis are also possible sequelae.