"Severitus" is a genre within the Snape/Harry genre: stories in which Snape is Harry's father. Sometimes, he adopts Harry, and sometimes, it is dicovered that Harry is his son. Either way, these stories are almost always gen, and are invariably hurt/comfort. Most tend to be very rambling, which is why you'll find a number of WiPs on this list. But if a paternal/filial twist on the snarry tropes appeals to you, read on:
E. M. Snape, In Blood Only (Rather dark, but well-done.)