While a properly vaccinated dog has some immunity from infectious diseases, there is still a possibility of contracting a fatal disease. Free-range dogs are at the highest risk, mainly from other dogs, but also from resident wildlife. Dogs who roam at will are also rarely up to date on vaccinations.

Parvovirus
A highly contagious viral infection passed through the feces of infected dogs. Known to be extremely fatal, Parvovirus is characterized by symptoms such as vomitting, diarrhea, and lethargy.

Parainfluenza
A respiratory infection that is passed from an infected dog's sneezing and coughing.

Rabies
Rabies is 100% fatal viral disease once symptoms appear. Contracted by contact with an infected animal, rabies is a zoonotic disease that is transmittable and lethal to humans.

Leptospirosis
A bacterial disease passed on through the urine of an infected animal. Bacteria enters the host animal through mucous membranes or through breaks in the skin. This disease can cause the liver to malfunction, kidney failure, and other internal organ failure.

Distemper
A highly infectious viral infection, spread through nasal secretions, sneezing and coughing from an infected animal. This disease has no cure and causes seizures, heart failure and death.

Free Range Dogs
Part 1: The Perils of Letting Your Dog Roam Loose in the Neighbourhood. Part 2: http://dogs.about.com/library/weekly/aa090102b.htmParasites Your Dog can Catch if He Runs Loose Part 3: Infectious Diseases Part 4: Aggravated People