The easiest way to keep your dog healthy is a high quality commercial diet. Generally speaking, most quality (and we emphasise quality) canine diets have the correct balance of everything your dog needs.
In addition, vaccinations help prevent the major diseases. However, for older dogs and certain breeds one-off problems can occur in dogs. More often than not this happens when he/she eats something he/she shouldn't have.
Older animals are most at risk because as your dog ages his/her alimentary canal becomes less efficient and that is when dog supplements and particularly dog vitamins can really help.
For dog supplements it is worth considering a probiotic, specially for older dogs. Probiotics contain a lot of "friendly" bacteria that help to break down food in the gut. Probiotics can also help reduce wind. Avipro Plus is one of our most popular dog supplements although others are available.
In addition to obvious infections such as E.coli and Salmonella inappropriate bacteria can overwhelm the gut of stressed, sick or elderly animals and cause diarrhoea. Probiotics are useful for helping to stabilise the gut while the natural bacteria recover.
When it comes to dog vitamins most dogs do not need a lot of vitamin supplementation however products like Zolcal D provide a combination of Calcium and Vitamin D which are useful to dogs. Vitamin D especially is important because dogs cannot synthesise Vitamin D so well compared to us. We humans have only to walk in sunlight to synthesise Vitamin D. But for dogs (with their fur) this is harder and supplementation is recommended.