The diseases we vaccinate against in dogs include canine parvovirus, canine distemper, infectious canine hepatitis (canine adenovirus) and canine parainfluenza virus.  Furthermore in certain endemic areas vaccination against leptospirosis is common.

Canine parvovirus is a highly infectious, often fatal disease of young dogs.  The virus is very resistant in the environment and can persist for many months if not years.  Symptoms include vomiting and bloody diarrhoea, rapid dehydration and death.  Parvovirus attacks not only gastrointestinal system but affects the immune system too, so affected pups may succumb to secondary infections.

Canine distemper is another very contagious virus of dogs, with clinical signs including ocular and nasal discharge, pneumonia, vomiting and diarrhoea and ultimately neurological signs and death.

Infectious canine hepatitis, also a disease predominately of young pups, attacks the liver and gastrointestinal system.  Affected animals show vomiting and diarrhoea, very rapid dehydration and may die.

I consider the above three disease the "core" viruses against which vaccination is essential.  Contrary to rumour, these vaccines are extremely safe and side effects are remarkably rare. Because these viruses affect young pups, generally between the ages of six and twenty weeks, starting the vaccination program early is critical.  I believe the first shot should be at around six to eight weeks old with a booster at twelve weeks.   In terms of frequency of vaccination after the initial course a second booster twelve months after the initial course is recommended.  After this frequency of vaccination would depend on individual circumstances including the level of likely challenge, the type (and registration criteria) of the vaccine being used and possibly the antibody titres of the animal.