Many people who have long COVID—a condition in which health issues persist months after infection—report struggling with “brain fog,” recurring memory and concentration lapses that make it difficult­­ to function in everyday life. Now a new study has found these cognitive problems could result from blood clots triggered by infection, possibly through mechanisms like those that cause some types of dementia. These clots leave telltale protein signatures in blood, suggesting that testing for them could help predict, diagnose and possibly even treat long COVID.