It all started in the small town of Lyme, Connecticut. For years, doctors had been diagnosing children with a type of juvenile rheumatoid arthritis that had begun popping up in the area. But it wasn’t until 1975 that the condition was conclusively identified as something separate, and named after the town where it was first observed.

In the decades since this discovery, the tickborne pathogen has spread to a number of new states and into Canada. In the U.S., Lyme disease is (by far) the most common tickborne illness, with the CDC recording around 30,000 cases annually. What’s more, not all cases are reported, and the agency estimates that as many as half a million Americans are diagnosed with the disease each year.

Let’s dig into the science behind how — and why — Lyme disease has managed to spread so far.

What Is Lyme Disease and How Do You Get It?

In humans, Lyme disease is caused by several related corkscrew-shaped bacteria called spirochetes. Most cases in North America are caused by the bacterium Borrelia burgdorferi, while B. mayonii, on rare occasions, is responsible for infections in the Midwest. The natural reservoir for these bacteria are small mammals like the white-footed mouse.

For a human to become infected, an infected tick has to bite them. Since ticks only feed a few times in their lifespan, it also means that a tick must have fed on an infected mouse in an early stage of its life before biting the human. Blacklegged ticks (commonly known as deer ticks) are the culprits that spread this disease in the U.S.

Once infected, humans sometimes develop circular rashes on their body. Other symptoms include headaches, fever and fatigue. When untreated, the disease can spread to the heart and nervous system. It can also cause facial palsy and arthritis if it seeps into the joints.

Can Lyme Disease Kill You?

While such incidents are extremely rare, the pathogen can be fatal: When Lyme disease bacteria enters heart tissue and interferes with the organ’s normal electrical signals, it can result in a condition known as Lyme carditis. According to the CDC, there have been 11 fatal cases of Lyme carditis, worldwide, between 1985 and 2019.

Still, cases of Lyme are seasonal, to a degree. “They peak in late June nationwide,” says Kiersten Kugeler, an epidemiologist with the U.S. Centers for Disease Control and Prevention’s (CDC) Division of Vector-Borne Diseases.

Read More: What You Need to Know About Lyme Disease

Where Did Lyme Disease Come From?

It’s not clear exactly how long Lyme disease has been around in the U.S. It also isn’t clear whether the bacteria originated somewhere in New England or whether it was imported from overseas.

But in more recent years, researchers have analyzed ticks kept in natural history collections dating back to previous centuries, revealing the presence of the bacteria in the Northeast. Other patient case reports from Wisconsin — a seemingly separate epicenter of the disease — in the past seem to show symptoms consistent with Lyme disease, says Kugeler.

The CDC first began to track Lyme disease in 1990, around the time that state health agencies also began to track the disease in a more systematic manner. Their efforts revealed that the disease spread from Connecticut into neighboring states, reaching northwards into Maine and southwards to North Carolina, Virginia and even into eastern West Virginia.

How Common Is Lyme Disease Today?

Similar tracking efforts by Canadian agencies show that the disease is making inroads north of the border, mostly in southern Ontario. There are also some pockets of the disease in Northern California, spread there by the western black-legged tick.

In addition to an expanding geographical range for ticks, more people are also getting infected: An analysis of insurance claims revealed that an estimated 329,000 people were diagnosed with Lyme annually between 2005 to 2020. A similar analysis by Kugeler and her colleagues showed that about 476,000 people were diagnosed between 2010 and 2018. Cases in Canada, meanwhile have increased from just 144 in 2009 to a whopping 3,147 in 2021.

Read More: Zoonotic Diseases That Have Been Transferred to Humans

How Does Lyme Disease Spread?

A number of factors may be responsible for the spread of Lyme disease. It mostly depends on the spread of the small mammals that are the primary reservoir for the bacteria. The disease is also dependent on the range of black-legged ticks — though Kugeler notes that ticks in the Southeast don’t seem to spread the disease as much, perhaps because they don’t feed as much on the small rodents that carry the bacteria in this region, preferring other hosts.

One of these causes is linked to changes in land use patterns, Kugeler says. Reforestation, for example, has created favorable habitats for the transmission of tickborne pathogens in many parts of the country.

“Suburban development in these areas has brought people, deer, rodents, ticks, and the germs ticks carry into close contact with each other,” she says.

Climate change is another driver. “We expect that changes in seasonality and location may occur,” Kugeler says, adding that new climate patterns can change longstanding ecological relationships.

“We don’t know what those changes will be,” she adds. “But we know that climate is only one of several very important factors that influence the distribution and occurrence of tickborne diseases.”

How to Avoid Lyme Disease

One of the main problems with Lyme disease involves a lack of education, especially in areas where the bacteria is just starting to make inroads. As a result, it’s critical to get doctors, and the public at large, informed about symptoms, including fever, rash, facial paralysis and arthritis.

“Education is our biggest weapon against Lyme disease,” Kugeler says, adding that individuals can protect themselves by checking for ticks after being outdoors, and by quickly removing any that have latched on.

Repellent can also help, as can treating your yard with acaricides — a type of pesticide focused on ticks and mites.

Read More: 450,000 Americans May Have A Meat Allergy Caused By Tick Bites