Peter and Shannon McDonald and their 9 children on their farm. Courtesy Peter McDonald.

Reported by Sharyn Alfonsi and Beth Loyd near Romulus, N.Y.:

Peter and Shannon McDonald have made their home on a farm with their nine children.

They are surrounded by farm animals - filthy ones, including chickens and slobbering pigs - but the eight boys and one brave girl have always been in perfect health.

It turns out that in a culture obsessed with cleanliness and anti-bacterial lotion, the animals may be one of the reasons these kids are so healthy.

"I live with dirt," Peter McDonald told ABC News. "We don't care about cleaning out the dirt. Many times they will eat it. I tell them to wash their hands only if there are big chunks of mud or dirt or who knows what on their hands, they don't care, they just do what they do. Dirt is our friend."

A study in the Journal of Allergy and Clinical Immunology has found that some non-farm families are diagnosed with asthma nearly twice as much as their farm-dwelling counterparts. The results for hay fever were even more pronounced, with nearly four times as many non-farm family members diagnosed.

Animals in general can be beneficial. A study published last year in the Journal of Pediatrics suggests that babies raised around dogs are 31 percent more likely to be healthy. Cats were shown to improve a baby's health by 6 percent. The research suggests that exposure to pets may help children's immune systems mature faster, with animals helping them grow antibodies to better combat infections.

In addition to animals, studies show that having even a small backyard garden means you'll eat five times as many vegetables, which can dramatically decrease your risk of cardiovascular disease.

"For breakfast if you had a Snickers and a malted milk, that's not going to have the building blocks that you need," said Peter. "The whole concept I think we have is, every day, anyone can improve their diet."

The McDonald farm is also a musical one, and - wouldn't you know it? - research has shown this improves memory, immunity and even muscle tone. All of which keeps this family and their collective health in perfect harmony.