Bostons might be small in stature, but they're big on being best friends. For more than 100 years they've been bred to enjoy the company of people, making them ideal for animal-assisted work where their sole job is to put a smile on patients' faces in nursing homes, hospitals and other health care facilities.

A handful of national organizations, including Therapy Dogs International and Bright and Beautiful Therapy Dogs help train and register dog-handler teams, as well as oversee volunteer programs throughout the country.

Getting started usually requires some basic training on both ends of the leash. Becoming a registered Pet Partner team with Delta Society, for example, entails completing an eight- or 12-hour training workshop or an at-home study course. Handlers learn how to prepare for a visit, interact with clients and prevent disease transmission from animals to people. They also undergo a skills and aptitude test with their dog, given by a local evaluator.

Today, more than 10,000 registered Pet Partners teams operate in all 50 states and other countries, including Canada, Italy and Japan.