An increase in reported cases of a rare polio-like illness that mostly affects children is being investigated by Public Health England.
Acute flaccid paralysis (AFP) affects the nervous system, causing one or more of the limbs to become weak or floppy.
PHE said 28 cases had been reported in England. Typically, a "handful" of cases are reported each year.
The disease remains very rare but anyone who develops weakness in any limb should seek medical help, it says.
The majority of cases reported in England have been since September. There has also been a rise in reported cases in the US.
Dr Mary Ramsay, head of immunisations at PHE, said: "AFP is very rare. However, if you or your child develops weakness in any limb, you should seek medical care immediately so that appropriate testing and care can be given.
"We are investigating potential causes and working hard to build better awareness amongst health-care professionals about how to test and manage patients with AFP.
"We are ensuring up-to-date information is available for patients and their families who may be affected."
Certain viruses are known to cause AFP, including polioviruses and non-polio enteroviruses.
Enteroviruses commonly cause mild infections with a range of symptoms, including colds, coughs and diarrhoea.
Such illnesses from viral infections are common, especially in children, and most people recover.
A type of enterovirus called EV-D68 and other viruses have been detected in several cases of AFP so far in 2018.
But PHE stresses that the risk of developing neurological symptoms due to a viral infection is extremely low.
AFP can be difficult to diagnose.
Doctors will typically examine a patient's nervous system and look at images of the spinal cord and brain.
They can also test the fluid around the brain and spinal cord and may check the nervous system conduction.
Doctors should report any suspected cases of AFP to Public Health England and samples should be sent to specialist labs for additional testing.