About the AU
The “virus” is highly contagious through physical contact. (But only if the user infected has any intention of infecting said person.) What it does is basically make anyone infected happy and energetic, giving them powers that they mostly use for ridiculous things like changing things to candy or floating around. They also use these powers to go to different dimensions to infect more monsters/humans.
How it works
BB takes any negativity found in the user and bottles it up, bounding it deep inside them along with the user them self. They are replaced with a much happier version of themselves, their outfit turning into something brightly colored and flashy. (sometimes striped or polka-dotted) BB’s goal is to infect as many monsters/humans as possible. The more he infects, the stronger the virus gets and the more likely he will successfully infect someone. Us!Sans himself is the main vessel of the virus, having been the first one to be infected.
Negativity is what makes it harder for the virus to infect. It takes more power and energy to infect someone with a lot of negativity than it is someone with less. Since BB was able to infect an entire dimension, he was able to take all of the negativity from Nightmare!Sans and bottle it up just barely. But, since NM was ONLY negativity, he is now, more or less, unresponsive and very slow to react. BB uses the negativity to power the virus, which is why he does prefer a challenge when encountered with someone who obviously isn’t the happiest.
Those who are infected with high negativity levels experience high levels of exhaustion and stress, stuck in a dark void similar to what US!Sans was in. Those with low negativity levels don’t experience this much. They are in a sleep-like state when they are infected.
The cure has to be extracted from a human SOUL. Though, too much could cause an infected monster to melt. If a monster were to come into contact with a human with the right SOUL, the infection in them will temporarily seize back. But once contact is removed, the infection will return. Those with a SOUL that can cure the virus can’t be affected.
Of course, with US!Sans being the main vessel of the virus, it really pays a toll on him. The virus absorbs the negative energy from others, which is also going into US!Sans. Not only is this extremely exhausting, but it’s painful and horrific. The virus’ true intentions for wanting to infect everyone are unclear, but they are most definitely not friendly.