For decades, April showers and May flowers have been followed by a burst of springtime tick activity, as an army of hungry parasites march through suburban backyards and rural areas in search of blood—maybe yours. As the climate warms, scientists are seeing earlier and greater tick activity in an expanded geographic range, which means humans, pets and livestock are at increasing risk of contracting a growing array of tick-borne diseases.

Lyme disease, the best-known illness caused by a tick bite, has been spreading from its 1970s range in the northeast U.S. to the rest of the country and Canada. Between 2000 and 2015, the number of Lyme disease cases in the U.S. doubled. Canada is just beginning to grapple with the problem: It reported 144 cases in 2009 and 987 cases in 2016, a six-fold increase. Yet, unreliable testing and treatments make it difficult to form an effective response. Many people who contract the virus suffer for years without an effective treatment.

“I grew up about an hour and a half from Lyme, Connecticut, in the 1980s when the Lyme epidemic was being recognized, and I remember the people who were sick for years and years without treatment because they weren’t showing symptoms that people first recognized as being associated with Lyme,” said Jeff Marcus, assistant professor of biology at the University of Manitoba, who has been studying the expanding range of ticks in Canada. “And it’s happening again in Canada, as the bacteria and the ticks are establishing themselves here.”

A single tick bite can put a human at risk for numerous diseases, not just Lyme. In 2017, Germany recorded 500 cases of tick-related encephalitis. Last year, Powassan virus emerged as a new threat, joining a list that includes babesiosis, anaplasmosis, Borrelia miyamotoi, Rocky Mountain spotted fever, ehrlichiosis, and tick paralysis. In March, the FDA approved testing the U.S. donor blood supply for babeseosis.

It’s not just an issue for people who live in the woods. In 2017, doctors in New York City reported several cases of Lyme disease in people who had not left the city, and the disease’s carrier, the black-legged tick, as well as two other ticks, are present in the city’s parks and carried throughout the city via mice, dogs and other mammals. Dogs are also vulnerable to tick-borne illnesses.

In North America, the primary carriers of illness are the black-legged tick, the lone star tick, the dog tick, the Rocky Mountain wood tick, and the tiny, terrifying deer tick. As if they weren’t bad enough, there’s a new one out there, too: In 2017, an infestation of the Haemaphysalis longicornis tick was discovered in New Jersey, heralding the arrival of a species that is native to East Asia, Australia, and several Pacific Islands. This creature is a carrier of numerous diseases and has a terrifying capability: It can reproduce without males, meaning one hitchhiker on a fruit shipment or global traveler is enough to start a whole new population.

Ticks have been called the “used syringes of the natural world” because they go through three stages in their lives and feed on a different host at each stage. They collect microbes in their blood meals, and transit them to the next mammal they feed upon.

Researchers with the WHO caution that ticks may play a role in the next pandemic. One tick-borne disease of special concern is Crimean-Congo haemorrhagic fever (CCHF), which has no available treatments and mortality rates as high as 40% in infected humans. Infected animals can maintain the virus in their blood at high levels yet not show symptoms, which means that livestock have the potential to carry it to global markets. The WHO says that CCHF, which was only discovered in 2009, can spread from infected ticks to humans, and from humans to other humans via bodily fluids. Uganda reported an outbreak in early 2018.

“[It is] important to screen the livestock for CCHF virus before the trade and transport takes place,” India’s National Institute of Virologydirector DT Mourya said, noting that India is one of the largest exporters of meat.Victims of CCHF have a burning fever, vomiting, and bleeding from their orifices.

Until the day tick-borne illness sneaks into us via meat, however, we can try to protect ourselves the old-fashioned way: with insect repellent and careful self-inspections of any suspicious bump or bite. (Look for the classic red bull-eye infection.) And take comfort: Not all ticks carry horrible diseases. Just the ones that do.