Whether it’s because summers are getting longer or we’re spending more time outdoors in nature, or simply because we’re testing more, it seems that the number of Lyme disease cases has been on the rise in the past decade. According to a new study, 14% of the world’s population currently has or has had tick-borne Lyme disease.

The dreaded ticks are parasites that feed off of the blood of mammals, birds, and sometimes even reptiles or amphibians. As if that wasn’t bad enough, they can also carry a lot of diseases. Borrelia burgdorferi sensu lato (Bb) infection, more commonly known as Lyme disease, is the most common type of tick-borne infection.

Diagnosing the disease can be very difficult. The most common sign of infection is a red rash that appears around the site of the tick bite about a week later. The bite and the rash are typically neither itchy nor painful, which makes it more difficult to diagnose — but the consequences can be severe. Early symptoms can include fever and headaches, and if untreated, Lyme disease can cause damage to facial nerves, paralysis, arthritis, heart palpitations, joint pains, and several other serious problems (including impaired memory). Even if treated, Lyme disease causes joint pains, memory problems, and tiredness in about 10-20% of cases. All in all, it’s a disease you don’t really want to get; which makes it even more surprising that so many people have it.

A team of researchers at Kunming Medical University in China looked at over 4,000 studies, selecting 137 of them and ultimately pooling data from 89 studies with almost 160,000 participants. The results suggest a global prevalence of Lyme disease of 14.5% — in other words, 1 in 7 people in the world either have the disease or have had it.

The highest prevalence was in central and western Europe (21% and 13.5% respectively) and in Eastern Asia (16%). In Southern Asia and the Caribbean, the prevalence was 2-3%. In North America, the rate was about 9%. The people most likely to get Lyme disease were over 50, males, and often lived in rural areas.

The results also suggest that the prevalence of Lyme disease in the 2011-2020 decade was higher than in 2001-2010, which could be linked to climate change, which could expand the preferred habitat of ticks (but more research is required to see exactly why this is happening). Lower rainfall, animal migration, fragmentation of habitats, and more time spent outdoor could also be factors at play.

The fact that so many people seem to have (or have had) the disease suggests that Lyme disease may be more prevalent than we thought. In fact, the researchers call for more long-term studies to assess exactly what is going on and assess just how big an impact the disease is having. A more accurate picture of the global distribution could “inform the development of public health response policies and [Lyme disease] control programmes,” they suggest.

Lyme disease is older than the human race and has been around for at least 15 million years. Ticks and the bacteria they carry are very opportunistic, and in many parts of the world (such as Europe or North America) they’re a more important vector of disease than mosquitoes. In the US, some 30,000 cases of Lyme disease are reported every year, but the real number is actually much higher.

Ironically, there was a vaccine approved for use for Lyme disease in 1998, but it was discontinued in 2002 due to “insufficient consumer demand,” according to the Centers for Disease Control and Prevention. Several treatments are also currently being researched.

The study has been published in BMJ.