The unofficial start of summer arrives this weekend — and ticks are wasting no time crashing the party.

Emergency room visits for tick bites jumped more than 25% nationwide in April compared to the same time last year, according to CDC data, with the Northeast getting hit especially hard as the tiny bloodsuckers swarm back for the season.

And those numbers only reflect people alarmed enough to seek care, meaning they’re likely just the tip of the iceberg.

“Tick season is here and these tiny biters can make you seriously sick,” Dr. Alison Hinckley, an epidemiologist with the CDC’s Division of Vector-Borne Diseases, said in a statement. 

The public health agency is urging Americans to stay vigilant when spending time outdoors, particularly as disease-carrying ticks steadily creep into new corners of the US.

By the fourth week of April, roughly 114 out of every 100,000 emergency department visits nationwide were linked to tick bites — the highest level for this point in the year since at least 2017.

The CDC estimates about 31 million Americans are bitten by the parasitic arachnids each year, with the number of tick-borne illnesses documented annually on the rise.

Here are five diseases you can catch if a tick sinks its teeth into you.

Lyme disease

With an estimated 476,000 Americans receiving treatment each year, Lyme disease stands as the most common tick-borne illness in the US.

The infection is spread to humans through the bite of blacklegged ticks carrying Borrelia bacteria. In most cases, the insect must remain attached for 36 to 48 hours or longer before transmission can occur.

Lyme disease can trigger a wide range of symptoms depending on how far the infection has progressed, according to the Mayo Clinic.

Early signs often include a bull’s-eye rash that spreads from the site of the bite, along with flu-like symptoms such as fever, chills, headache, fatigue and muscle and joint aches.

If left untreated, the bacteria can spread through the bloodstream, affecting the heart and nervous system and leading to symptoms including a spreading rash, facial paralysis, irregular heartbeat, nerve pain and numbness in the arms and legs.

Alpha-gal syndrome

This potentially life-threatening illness makes people violently allergic to the “alpha-gal” sugar molecule found in red meat, dairy and other mammal-derived products.

Most cases are linked to bites from lone star ticks, which can transfer alpha-gal from their saliva into the bloodstream, prompting the immune system to mistakenly identify it as a threat and trigger an allergic response.

Symptoms can include itchy skin, hives, stomach pain, nausea, vomiting, diarrhea, dizziness and fainting. In severe cases, it can cause difficulty breathing or anaphylaxis.

Researchers last year reported what is believed to be the first known death linked to the condition, involving a 47-year-old New Jersey pilot who became ill hours after eating a hamburger in 2024.

Once virtually unheard of in the US, cases are rising as lone star ticks expand beyond the Southeast into states like New York and Maine and farther west.

The CDC estimates as many as 450,000 Americans may have been affected as of 2023.

There is no currently no cure for alpha-gal syndrome. Management focuses on avoiding mammal-derived products.

Rocky Mountain spotted fever

This potentially life-threatening illness is spread through the bite of a tick infected with the bacteria Rickettsia rickettsii.

Early symptoms often look like a bad case of the flu and may include headache, fever and muscle aches, according to the Cleveland Clinic. Some patients also develop a rash that appears as flat red, pink, purple or dark splotches or dots.

If not treated quickly, the bacteria can attack blood vessels and damage muscles, nerves and organs, leading to leaking blood vessels, fluid buildup and widespread tissue injury.

Complications can be severe, including acute respiratory distress syndrome, kidney injury, fluid in the lungs, anemia, heart disease, internal bleeding, sepsis, partial paralysis, brain inflammation and, in extreme cases, tissue death requiring amputation.

When caught early, the infection can be treated with antibiotics, though complications can still occur even with treatment.

About 1,200 cases are diagnosed annually in the US, though numbers have been rising in recent years.

Anaplasmosis

A bacterial infection caused by A. phagocytophilum, this disease is spread primarily through the bites of blacklegged ticks and western blacklegged ticks.

Early symptoms are often flu-like and can include fever, chills, headache, muscle aches, nausea, vomiting and diarrhea.

People over 50, those with weakened immune systems or those who aren’t treated promptly face a higher risk of severe illness.

In advanced cases, anaplasmosis can lead to respiratory failure, internal bleeding, organ failure, brain inflammation and, in rare cases, death.

About 5,000 to 6,000 people get anaplasmosis every year across the country, according to Harvard Health. The infection can be treated with antibiotics if caught early.

Ehrlichiosis

This is a general term that covers a group of diseases spread through the bite of ticks infected with Ehrlichia bacteria.

Early symptoms often resemble the flu and can include fever, chills, headache, muscle pain, fatigue, cough, nausea, vomiting, stomach pain, confusion or changes in mental state and sometimes rash, according to the Cleveland Clinic.

If not treated quickly, ehrlichiosis can become severe or life-threatening, leading to respiratory failure, heart failure, kidney failure, central nervous system damage, sepsis, seizures and coma.

The infection can be treated with antibiotics, but early diagnosis is critical.

About 500 cases are reported in the US each year, though experts at Columbia University’s Irving Medical Center say the true number is likely significantly higher.

“The good news is you have options to help prevent tick bites when you spend time outdoors,” Hinckley said.

That includes wearing EPA-registered insect repellent and permethrin-treated clothing, doing tick checks and removing attached ticks as soon as possible.

“These simple steps can go a long way in protecting you and your family from diseases spread by ticks,” Hinckley said.

“And if you develop a rash or fever in the days to weeks after a bite, or after being in an area with ticks, seek medical care promptly.”