Zido might not have a stethoscope or blood pressure cuff, but he monitors vital signs, alerting Courtney Williams if her heart starts racing or her blood pressure drops.

The 8-year-old yellow Labrador retriever is trained as a cardiac alert dog. He spends 24/7 with Ms. Williams, commuting from their home in Lorton, Va., to her job in Washington, D.C., sitting under her desk as she works and tagging along on errands. He even had a reserved spot in the front row at her wedding.

Zido will step in front of Ms. Williams when she walks, offer a deep, purposeful lick or rub his nose on her leg if he needs to alert her to a change in her vital signs that puts her at risk of fainting due to an autonomic nervous system disorder.

"Before I got Zido, I was having one episode a week,” Ms. Williams, 26, told Medical Daily. “[In the 6 years] since we've been partnered, I've had 1 episode. There's never been a time when he's alerted [me] and been wrong."

Ms. Williams is one of 500,000 Americans with disabilities who have service dogs.

The Americans with Disabilities Act (ADA) defines service dogs as “dogs that are individually trained to do work or perform tasks for people with disabilities. ... Dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA .” Their roles can range from guiding those who are blind, pulling a wheelchair, alerting for low blood sugar or providing medication reminders.

Service dogs are carefully selected and undergo rigorous training and socialization that lasts between 6 months and 2 years. Most organizations train breeds such as Labrador retrievers, golden retrievers, German shepherds, poodles and mixes such as labradoodles and goldendoodles as service dogs, according to Michele Ostrander, president and CEO of Freedom Service Dogs of America . The ADA only recognizes dogs and miniature horses as service animals, she added.

These breeds are large enough to reach drawers and refrigerator handles, push elevator buttons or provide stability if their owners need help with mobility or support to get up after a fall. “These breeds also have the right temperament,” said Ms. Ostrander. “They are eager to please and like to work.”

When service dogs are off duty, they go for walks, play fetch, chew on squeak toys and snuggle with her owners. But unlike pets, service dogs have jobs to do.

Even therapy animals – such as the dogs, cats, rabbits, rats, miniature horses and llamas that are on the job during visits to airports, hospitals and nursing homes -- are not considered service animals.

“They provide comfort, bring smiles and have therapeutic benefits, but are not trained to do specific tasks,” Ms. Ostrander said.

Organizations often invest upwards of $30,000 to train service dogs, due to their intensive training and specialized skills. Nonprofit organizations like Freedom Service Dogs, Guide Dog Foundation and Canine Partners for Life raise funds through grants and donations, so their clients aren’t charged the full fees for their dogs.

The waiting lists for service dogs are long. At Freedom Service Dogs, clients wait up to 4 years to be matched. It’s also possible to purchase trained service dogs from private trainers. The fees are substantial, but the experience of having a service dog is priceless.

Ms. Ostrander partnered a military veteran with post-traumatic stress disorder with a service dog. The veteran, once afraid to leave the house, took his family on a Disney vacation. In another family, a service dog helped a son with autism feel safe enough to sleep in his own bed for the first time in his life.

“Service dogs truly transform lives,” she said.