Some COVID-19 symptoms could be the result of the virus crossing the blood-brain barrier, new research has found.
It stops most pathogens, but the barrier doesn't seem to protect against the virus' spike proteins, new research on mice suggests.
SARS-CoV-2's spike protein - the sticking-out bits you see in pictures, also known as S1 - can detach from the rest of the virus and sneak through the blood-brain barrier, scientists at the University of Washington found.
"The S1 protein likely causes the brain to release cytokines and inflammatory products," said study lead author William A Banks.
A cytokine storm is when the immune system overreacts in its attempts to kill an intruder, causing damaging inflammation.
Dr Banks said it was like "deja vu", having seen the HIV virus - which causes AIDS - do the same thing. He said the finding, if transferable to humans, could explain many of the stranger symptoms of COVID-19 - such as delirium, psychosis and even Parkinson's.
"We know that when you have the COVID infection you have trouble breathing and that's because there's infection in your lung, but an additional explanation is that the virus enters the respiratory centres of the brain and causes problems there as well.
They also found the virus was quicker to get into the brain, via the nose, in men than women - perhaps explaining why the disease is deadlier for men.
"You do not want to mess with this virus," said Dr Banks. "Many of the effects that the COVID virus has could be accentuated or perpetuated or even caused by virus getting in the brain and those effects could last for a very long time."
Around one in 45 people who fall ill with COVID-19 go on to have symptoms lasting several months, a condition known as Long COVID. As a new disease, it's unknown how long the condition could last.
The research was published in journal Nature Neuroscience this week.
