If it’s summer, people on the East Coast, in the Gulf states and in the upper Midwest are worrying about deer ticks. That's because these poppy seed-sized arachnids (yes, technically they're spiders), carry Lyme disease. They're most active when summer drifts into fall, but they're happy to hitch a ride on a passing warm-blooded creature as early as the the spring thaw.

Related: 6 Ways to Prevent Lyme Disease

But deer ticks aren't the only ticks to watch out for. And Lyme disease isn't the only disease ticks can carry. It turns out that other infections are competing for the title of most fearsome disease carried by a tick. Here's what you need to know about the contenders.

Rocky Mountain Spotted Fever

Rocky Mountain spotted fever, which is caused by the bacterium Rickettsia rickettsia, may handily win the title for worst tick-borne disease. If left untreated, it wreaks havoc on internal organs, including the kidney, heart and brain, where it can cause encephalitis (a brain inflammation). Most alarming, the disease can be fatal if not treated within the first few days of infection.

Symptoms. The initial symptoms of Rocky Mountain spotted fever resemble those of a terrible cold or flu. They include a high fever, chills, severe headache, stomach pain, vomiting, restlessness, insomnia and muscle pain.

Unfortunately, the most tell-tale symptom of Rocky Mountain spotted fever doesn't effect everyone who comes down with it: a splotchy or spotty red rash that first shows up on wrists and ankles, soles of feet and palms of the hand. It begins as small, flat and pink spots that don’t itch and that turn white when you press them. 

Where it shows up. Rocky Mountain spotted fever first appeared in the Rocky Mountain region (hence the name), but it crops up most frequently in southeast. It’s also seen in parts of Mexico, Canada, Central American and South America.

The disease is transmitted by the American dog tick, the Rocky Mountain wood tick, and the brown dog tick. The American dog tick (see photo above) is found wherever there are dogs — that is, pretty much everywhere. And since American dog ticks live comfortably in kennels, barns and houses all year long, their season is whenever they find a source of nutrients. That’s why you may occasionally pick a tick off your pooch in the middle of winter.

People who live around dogs, woods and areas with tall grasses are at high risk of being bitten by a tick carrying Rocky Mountain spotted fever. Over a recent seven-year period, 140 cases of Rocky Mountain spotted fever were reported in an area of eastern Arizona where the infection had never been seen before. About 10 percent of people diagnosed with the disease died. As reported by the Centers for Disease Control and Prevention, “almost all of the cases occurred within communities with a large number of free-roaming dogs.”

Prevention. To prevent Rocky Mountain spotted fever, take the same precautions you would take to ward off Lyme disease (in a word, do everything you can to avoid the ticks waiting for you at the tips of the grass or branches, waving their tiny front claws in the air until something brushes by that they can latch onto.) For advice on removing ticks from you, your children or pets, click here.

Treatment. If you're bitten by a tick or develop symptoms, seek medical treatment immediately. Specialized tests can confirm (or rule out) the diagnosis, and the disease usually responds well to antibiotics.

Related: 10 Health Symptoms Women Should Never Ignore

Babesiosis

Babesiosis, which is caused by the parasite Babesia microti, produces symptoms similar to malaria. It may cause only a mild illness, but it can be fatal if untreated.

Where it shows up. Babesiosis is found in 27 states, mostly in the Northeast and upper Midwest. Ninety-five percent of the cases occur in Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island and Wisconsin.

Symptoms. Some people infected with the bacteria have no symptoms, while other develop the usual flu-like symptoms of tick-borne infections, including fever and chills, fatigue, headaches, body aches, sweating, loss of appetite and nausea.

The nasty twist is that Babesia parasites target red blood cells, destroying them and sometimes causing hemolytic anemia, which can cause the skin to yellow and result in an enlarged liver and other complications.

Babesiosis can be a life-threatening disease, especially in people who have a weak immune system (such as cancer or AIDS), who don’t have a spleen, who have liver or kidney disease or who are elderly. Its complications include unstable blood pressure, severe anemia, a low platelet count, bleeding and blood clots, organ failure and even death.

Prevention. Avoiding tick habitats is the best prevention, especially for people who don’t have a spleen and are more vulnerable to a severe infection. If you like to hike and spend time in the woods, take the same precautions as you would with Lyme disease.

Treatment. Diagnosing Babesiosis is quite simple. A blood test allows medical providers to look for  parasites inside red blood cells. If you have the parasites and are showing symptoms, your doctor will prescribe an effective treatment — typically, either atovaquone and azithromycin or clindamycin and quinine.

Related: Another Tick-borne Illness Documented In The Northeast

Borrelia miyamotoi

Although not yet affecting enough people in the U.S. to be a real contender, a little-known disease carried by the same tick that transmits Lyme disease is flexing its muscles. It’s also carried by the western black-legged tick (Ixodes pacificus).

Where it shows up. The rare bacterium Borrelia miyamotoi has caused alarming illnesses in the Northeastern states, including a life-threatening brain infection called meningoenceophalitis.

Symptoms. Like Lyme disease, which is known as “The Great Pretender” because its symptoms mimic those of many other diseases, Borrelia miyamotoi causes headache, fever, chills and muscle pain. A rash is seen in only about 10 percent of patients. Repeated bouts of fever are the hallmark of the disease.

This infection is quite new. Researchers first learned about the tick-borne bacterium in 2001. A 2011 report from Russia reported the first human cases, and an elderly woman who fell ill in the Northeast in 2013 became the first case to be diagnosed in the United States. Only three U.S. cases have been confirmed to date.

Prevention. The tick-fighting advice is the same for most of these blood-sucking pests: Avoid tick habitats and check yourself if you do go in a tick-infested area. Since a tick has to stay attached to you for 24 hours to infect you, checking yourself, your children and pets for ticks and removing them immediately will prevent Borrelia miyamotoi (and many other tick-borne diseases).

Treatment. Nearly 25 percent of people with the disease need to be hospitalized, but the disease responds well to treatment.

Related: Could You Have Chronic Lyme Disease?

Denise Foley is a veteran health writer and a former contributing executive editor at Prevention magazine.