Some ticks carry pathogens like viruses and bacteria that enter the bloodstream of the animals and people they bite.
Ticks are small, blood-sucking arthropods, and like their cousins - mites, spiders and scorpions - they have eight legs. There are many different species of tick, with different ones biting and sucking the blood of different animals, and sometimes this includes humans.
Some ticks carry pathogens like viruses and bacteria that enter the bloodstream of the animals and people that they bite. There are many different tickborne illnesses caused by a range of pathogens.
The Centers for Disease Control and Prevention (CDC) list at least 14 different types of tickborne diseases known in the US.
Tickborne diseases usually cause fever, chills, aches, pains and rash. Symptoms range from mild reactions that are treatable at home to severe infections requiring hospitalization.
'Entirely new species of bacteria'
The new discovery, reported in The Lancet Infectious Diseases, is the work of a team of researchers from China and the US. In their paper, they say it is possible that the newly-discovered disease could be a "substantial" threat to human and animal health in the region where the tick prevails.
They name the newly discovered pathogen - a bacterium - Anaplasma capra, after the fact it appears to be common in goats. "Capra" is the Latin word for "goat."
The bacterium is related to other Anaplasma bacteria, some of which can also cause illness when transmitted from ticks to humans.
However, the researchers note they are not sure how widespread A. capra and the tick that carries it might be and whether they bite other animals as well as goats.
Co-author J. Stephen Dumler, a professor of pathology at the University of Maryland School of Medicine in Baltimore and an expert with global experience of tickborne diseases, says:
"This is an entirely new species of bacteria. This had never been seen in humans before. We still have a lot to learn about this species, but it may be that this bacteria is infecting humans over a wide area."
Prof. Dumler himself discovered another Anaplasma bacterium that causes the disease human anaplasmosis 2 decades ago.
For the study, he and his colleagues - including researchers from the Beijing Institute of Microbiology and Epidemiology, the Mudanjiang Forestry Central Hospital and Shanghai Jiaotong University, all in China - tested 477 patients in northeast China who had been bitten by a tick over the period of a month in the spring of 2014.
They found that 6% of the patients - 28 individuals - were infected by the new species of bacteria - A. capra.
A. capra probably transmitted by the taiga tick
Not much is known about A. capra. It is not easy to diagnose - there is no simple blood test.
The researchers say A. capra is probably transmitted by the taiga tick - a close relative of the deer tick. The tick is widespread in Eastern Europe and Asia - including Russia, China and Japan.
If the taiga tick spreads A. capra throughout this region, then human infection may be common, says Prof. Dumler, who notes that more than a billion people live in areas where the tick is prevalent. He and his colleagues conclude:
"The emergence of A. capra as a cause of human disease suggests that individuals living in or traveling to endemic regions in northern China should take precautions to reduce their risk of exposure to this novel tickborne pathogen."
The Natural Science Foundation of China and the US National Institutes of Health funded the study.
In 2013, Medical News Today reported how researchers from Yale University found a new infection caused by ticks that is extremely similar to Lyme disease. They described how they found the unnamed disease for the first time in humans - 18 people in southern New England and upstate New York - in the New England Journal of Medicine.