The cuddle bug: an other-dimensional sickness, one that presents with flu-like symptoms and mental distress unless the afflicted individual is in physical contact with another person, which calms the sickness. Treatment is consistent and prolonged physical contact with another to aid in recovery.
Ford contracts the cuddle bug during his time dimension hopping, and comes down with the illness shortly after Stan successfully reopens the portal and brings him back home. Ford is aware of what the cuddle bug is after having encountered it before, but is hesitant to tell his family about it.
Eventually, Dipper notices Ford getting sicker and sicker, and manages to get the truth out of him. Dipper then offers to help Ford get better, and also tells Mabel. Mabel is always down for cuddles and willing to help, but sees an opportunity to get her uncles to bridge the gap between them.
So Mabel tells Stan, who doesn’t believe her at first. He’s only convinced once he sees the proof of how ill Ford has gotten, and Mabel and Dipper work to convince Stan to help Ford get better. Stan is against it and Ford is against it, but the kids are persistent. And in the end, Stan would do just about anything for his brother.
Cue family cuddle pile to ease Ford’s symptoms and help him recover from the illness.
Ford contracts the cuddle bug not long after he begins exploring Gravity Falls. At first, he has no idea what the illness is, because it can’t be found outside of Gravity Falls. He slowly gets sicker and sicker and medications are not helping. It’s a gnome or some anomaly that eventually informs him about the cuddle bug.
Ford theorizes the bug is an other-dimensional sickness that slipped between the cracks into Gravity Falls. Hearing about what the apparent cure for the illness is makes him scoff. Surely he doesn’t need to cuddle with anyone to get over it. That part is probably made up.
Except he gets sicker. And sicker. And his mental health deteriorates. He can’t work in his condition, and despite all his studying about the bug, Ford has not found any sort of cure. Growing desperate to get better, Ford finally caves and decides to try the physical contact route.
Except he doesn’t know anyone in town, and the one friend he made in university just got married. He debates calling his mother, but she’d never be able to come out to Gravity Falls on her own, his father would come with her. Ford does not want Filbrick to find out about any of this.
Really, in the end, there was only ever one person Ford would be willing to call about such an issue. It ends up being really not that difficult to get Stan to agree to come see him.
Stan is highly skeptical about this “cuddle bug” sickness Ford tells him about, but he hopes this could be a chance to reconcile with his brother. And it does turn out that way, after lots of arguing and yelling and cuddling. Ford finally starts to recovery with the physical contact, and their relationship starts to recover as well.