Read the Infection power in the core book.
When a critter with this power uses Essence Drain to reduce the victim’s Essence to 0, this power attempts to pass the critter’s own infection on to the victim.
Read HMHVV Strain I in Run Faster.
Vector: Infection power (p. 398, SR5)
This strain is responsible for creating banshees, dzoo-noo-qua, goblins, mutaqua, nosferatu, vampires, and wendigos. It’s only spread by the Infection power.
You can only be infected with Strain I or Ia by being Essence Drained to zero Essence. Otherwise you can not be Infected. Without being completely drained, you can only be a Strain I or Ia Carrier, aka have the Carrier negative quality, RF 141.