(Rachel Orr/The Washington Post)

Dear Science: Now and then health awareness articles use the attention-grabbing statistic "XYZ has more germs on it than a toilet seat!" Is this actually a useful comparison? Is the sheer number of germs what we should worry about? And how germy (and dangerous) is the seat, anyhow?

Here's what science has to say:

Every time a commercial for disinfectant trumpets the claim "kills 99.99 percent of germs," a trillion bacteria burst into hysterical laughter at the depth of human folly. There are few things less realistic — or misguided — than our attempt to eliminate germs.

"In the 150 years since we identified that bacteria were the cause of disease, we’ve become obsessed with the abundance of cells," said Jack Gilbert, an environmental microbiologist at Argonne National Laboratory. "The thinking is, 'bacteria equals disease therefore more bacteria equals more disease.' But that just isn't the case."

Scientists estimate that Earth is home to about 1 trillion species of bacteria. They dwell in the soil, the ocean, the air, your skin, your innards — they're unavoidable. Basically, it's the microbes' world, and we're just living in it. And according to David Coil, a microbiologist at the University of California at Davis, it's time we embrace that fact.

"They’re sort of the masters of the planet," he said. "Without bacteria much of what makes the world go ’round would grind to a halt."

Even if we could get rid of all the germs, we'd be sad to see them go. Bacteria fix nitrogen in the soil, allowing plants to grow. They break down biological waste and return nutrients to the ecosystem. The ones that live inside us help our digestion and boost our immune systems. They're also used to make cheese, chocolate, kimchi, beer, sourdough bread and coffee (making them indirectly responsible for this column, among other things).

So you're right to be skeptical about those "health" "awareness" headlines. The sheer number of germs on a given item doesn't really tell you much about how dangerous it is, because most bacteria aren't harmful to humans. In fact, there are plenty of circumstances where swallowing some bacteria could boost your health — hospitals are using fecal microbiota transplants (i.e. poop donation) to save the lives of people suffering from chronic gut diseases. Some researchers are even exploring poop transplants as a weight-loss method.

And a toilet seat is a terrible yardstick of germy-ness regardless. Coil has swabbed them "on multiple occasions," he said, "and I've found nothing too exciting." We use so much Lysol and the like on toilet seats that microbes don't have an opportunity to accumulate there; a spoonful of soil has more bacteria and far greater diversity.

If you're worried about bacterial infections, you need to focus on the types of microbes you're exposed to, not the amounts. We think of toilet seats as being dirty mainly because their association with just one species: Vibrio cholera, which is found in human feces and causes over 100,000 deaths a year. And if you lived in a place with poor sanitation infrastructure, unreliable access to clean water, and endemic cholera, you'd be right to worry about the bacteria in your bathroom.

But unless cholera or some other pathogenic bacterium is already present in your household, Gilbert pointed out, you're unlikely to get ill from touching your own toilet.

Indeed, our eagerness to eliminate bacteria from our world could be hurting our health. That's the gist of a theory known as the Hygeine Hypothesis, which proposes that our modern, sterile environment may be contributing to a rise in allergies and other autoimmune diseases

"Our ancestors grew up being exposed to rich microbiota on a regular basis, and our immune system expects to still see that," Gilbert said. "But we have severed our relationship with that world by moving indoors and through indiscriminate use of antibiotics [and] cleaning products."

Growing up without this exposure to microbes could make our immune systems go haywire; because they haven't been educated about the appropriate response to pathogens, they start treating everything as an enemy — fur, hay, peanut butter, etc.

This doesn't mean we should be encouraging our kids to go out and lick toilet seats (not that every kid needs encouragement).

"We have to be careful," Gilbert said. "But we also want to be very careful about how protective we become."