To pet a dog is to know peace.

But who’s this interloper?

That’s a brown dog tick.

They’re the most widespread tick in the world,  and the most adapted to living among us.

Brown dog ticks are thought to  have evolved alongside burrowing   carnivores like foxes and weasels – and  came indoors when we domesticated dogs.

They can be found in and around homes.

And what’s worse, they spread  bacteria that can be deadly.

They aren’t the ticks known for carrying  Lyme disease.

Those are blacklegged ticks.

The brown dog tick has grooves along its back,   and they’re a solid, reddish  brown.

See the difference?

No matter what kind of tick they  are, they want one thing: blood.

And to find that blood, they use what’s called the  Haller’s organ, one near the tip of each foreleg.

Ticks use them to pick up chemical signals from  the air: carbon dioxide, pheromones and humidity.

Scientists believe the Haller’s organ even  lets ticks detect the body heat of their prey.

All ticks have them, but  they use them differently.

The blacklegged tick “quests” – it stays put,   waving its forelegs to sense  when it can hop aboard a host.

The brown dog tick hunts, using that Haller’s  organ to home in on a potential target.

As its name suggests, a brown dog tick  is happy to take all its meals from dogs.

But in the right conditions, the  brown dog tick will dine on you, too.

That’s a problem, because they can transmit  bacteria that cause Rocky Mountain spotted fever,   a terrible disease that can  kill both dogs and humans.

Rocky Mountain spotted fever usually  occurs in small clusters in the United   States and is relatively rare.

But outbreaks in  northern Mexico have killed hundreds of people.

And rising temperatures due to climate change  are sparking some troubling tick behavior.

When it’s particularly hot out, brown  dog ticks start craving human blood.

To investigate this, University of California,  Davis researchers put a very good dog in a box   and a very good human in another, connected by a  plastic tube with hungry brown dog ticks inside.

Don’t worry – there’s a screen here and  here.

The ticks can’t actually get them.

At room temperature, the ticks preferred  dogs.

But when researchers heated up the tube,   to 100 degrees Fahrenheit, brown dog  ticks preferred – you guessed it – us.

Scientists are still trying to  determine why.

In the meantime,   researchers are developing vaccines  to protect us from the disease.

Tick treatments can keep the pests  off of dogs.

But they’re expensive.

In the Sonoran Desert, in Southern  California, volunteers remove ticks by   putting their tweezers right up against  a dog’s skin and pulling straight up.

This one is full of dog blood.

And they give the dogs oral medicine for free.

Look at these happy pals!

And then it’s back to the  petting frenzy you both deserve.