A worm has been found residing in a woman's head.

Surgeons in Arizona were looking for a brain tumour in Rosemary Alvarez's skull after she complained about blurred vision and numbness in her arm. However, a CT scan and a further MRI revealed something very different.

"Once we saw the MRI we realised this is something not good. It's something down in her brain stem which is as deep in the brain as you can be," said neurosurgeon Dr Peter Nakaji.

The doctor operated on Alvarez and pulled out a worm, a condition which can develop from eating undercooked pork and can be spread by not washing your hands after using the bathroom.

"It only takes one person who is spreading it constantly to get a lot of people exposed and some of those people are going to go on to develop this problem," he said.