A disease that can harm the brain and which is spread to humans by tick bites has been identified in the insects in Britain for the first time.
Public Health England (PHE) has confirmed the presence of the tick-borne encephalitis virus in Thetford Forest and on the Hampshire-Dorset border.
PHE says the risk is "very low", but it is investigating how common the infected ticks might be.
Ticks are becoming more common in parts of the UK, mainly due to increasing deer numbers.
They can also live in other animals, such as dogs, and in undergrowth.
Ticks can also carry other diseases, including Lyme disease.
However, being bitten by a tick does not necessarily mean a person will fall ill.
The tick-borne encephalitis virus is already present in mainland Europe and parts of Asia.
It is thought infected ticks may have arrived in the UK through migratory birds.
Dr Nick Phin of PHE said: "These are early research findings and indicate the need for further work. However, the risk to the general public is currently assessed to be very low."
Most people who catch the encephalitis virus will have no or only mild flu-like symptoms.
But it can affect the brain and central nervous system and can sometimes be fatal.
"We are reminding people to be 'tick aware' and take tick precautions, particularly when visiting or working in areas with long grass such as woodlands, moorlands and parks," Dr Phin told the BBC.