Scientists believe they can explain why some people who catch Covid develop chilblain-like lesions on their toes and even fingers.
Covid toe appears to be a side effect of the body switching into attack mode to fight off the virus.
The researchers say they have pinpointed the parts of the immune system that appear to be involved.
The findings, in the British Journal of Dermatology, may help with treatments to ease the symptoms.
It can happen at any age, but affects children and teenagers more commonly.
For some it is painless, but the rash can be extremely sore and itchy, with tender blisters and swelling.
Sofia, who is 13 and from Scotland, was barely able to walk or wear shoes when she developed Covid toe earlier this year.
Media caption,’My life living with Covid toes’
Over the summer, she told BBC Scotland’s The Nine how she relied on a wheelchair for longer walks.
Covid toes left me unable to wear shoes
Covid toe and other rashes puzzle doctors
The affected skin – usually the toes, but sometimes the fingers – can look red or purple. Some people develop painful raised bumps or areas of rough skin. There can also be pus