In fanfiction, Snape's reaction could be complex. He may recall his own past traumas and experiences with difficult situations. This could lead him to a moment of hesitation, but ultimately, his sense of duty might prevail. He could send for help, like Madam Pomfrey, while trying to keep the other students at bay. He might also start to analyze what could have caused the seizure, using his sharp intellect and knowledge of magic, all the while having a conflicted look on his face as he deals with his mixed feelings towards Harry.
Snape might initially be suspicious. He has always had a distrust of Harry, so he could think it's some sort of trick. But then, as a Potions master and a teacher, he might also feel a sense of responsibility and try to help.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
One reason could be that Harry is under an extreme amount of stress. He has so many burdens on his shoulders in the wizarding world, like fighting Voldemort. This stress could manifest physically as a seizure, especially if he is in a tense situation in front of Snape, who is often a source of anxiety for him. For example, if Snape is about to accuse him of something he didn't do while he is already worried about other threats to the wizarding world.
In this fanfiction, the seizure could be part of a curse that was meant for Sirius but backfired on Harry. Harry starts seizing in front of Sirius, and Sirius is filled with guilt and determination to reverse the curse. He uses his knowledge of the wizarding world and his connections to search for a cure, all while taking care of Harry and keeping him safe from any potential threats.
Snape would probably be very conflicted. On one hand, he has a long - standing grudge against James Potter, Harry's father. But on the other hand, he loved Harry's mother, Lily. So, he might be cold at first, but then his sense of protecting Lily's son could kick in. He would likely be very strict with Harry, as he is with all his students, but also secretly keep an eye on him to make sure he's safe.
Snape could be initially shocked. He's used to seeing Harry as a rather tough and rebellious student. So, seeing Harry cry might take him aback for a moment. Then he might become a bit more guarded and try to figure out if Harry is faking it or not.
It could be a new dark wizard or witch who has a vendetta against Harry. This person might have developed a new form of magic specifically to harm Harry, and the seizure is the first sign of this new threat.
Snape would likely be angry at first. Harry has always been a source of trouble in his eyes, and running away would just add to that perception. He might mutter some sarcastic remarks under his breath about Potter's irresponsibility.
Snape might be initially taken aback. He could hide his surprise behind his usual stern facade and perhaps try to maintain order, but secretly be a bit worried.
The Slytherin house would probably be outraged. Snape is one of their own, in a sense. They would demand justice for Snape and might even use this incident to fuel their long - standing rivalry with Gryffindor. There would be a lot of tension and conflict within the Hogwarts community as a result of this act.
Harry would probably be quite resentful at first. He might think Snape is just being his usual unfair self. So, he would be sulky and do the tasks reluctantly.