[Maureen]: He senses whenever we are going to go out,

and he knows before we even make a move.

[narrator]: They read our body language

and that's how they know we're going to take them for a walk

before we think we've made the decision ourselves.

It's this constant observation that can sometimes seem to give them a sixth sense.

Max is a red collie cross, normally full of energy and enthusiasm.

But in 2008, his owner, Maureen Burns, noticed that something was wrong.

[Maureen]: I thought at the time Max was fading. He was nine and a half.

and I was preparing myself for losing him, because of... he just wasn't happy.

Wouldn't come by me, wouldn't sit by me, wouldn't sit on my lap.

And the odd signs, he would come up, and touch my breast with his nose

and back off, so desperately unhappy, with such a sad look in his eyes.

[narrator]: Maureen had a small lump in her breast,

but her latest mammogram was clear, so she presumed all was OK.

Initially I thought, it's just another lumpy breast, as people get.

But then, I sort of connected it with the dog. With his odd behaviour.

And one day, I was looking in the mirror, in the bedroom here, and I looked across in the mirror at Max's eyes.

He was on the bed, as he is now, and I knew it was cancer.

[narrator]: Maureen went to her local hospital, but both a scan and a mammogram came back negative.

It took a surgical biopsy to finally detect the cancer.

Maureen had it removed, and the change in Max's behaviour was instant.

[Maureen]: The day I was picked up from the hospital, he was his old hyper self again.

Put his nose across my breast, to check where the operation had been, and he was wagging his tail and his eyes were happy.

It was unbelievable, the change in him. Instant.

I love Max so much, and I owe him so much.

Even before I go for a checkup now, I get him to check me out.

And if he's happy and wagging his tail, I'm happy, and I know I'm OK.

[narrator]: We now know that dogs like Max are smelling the tiny volatile chemicals given off by cancerous tumors.

Most dogs can do this, they just don't know that we're interested.

But with a little training, dogs can accurately pick out a cancerous patient from a selection of urine samples.

[sound of clicking] [woman]: Good girl.

[narrator]: They can even be taught to let diabetics like Steven know that his blood sugar levels are low, long before he can feel it,

and definitely before he passes out.

Dogs like Steven's are now transforming lives all over the country.

They're being trained to detect the early warning signals for severe allergic reactions, epileptic fits, and narcolepsy.

[lively music]