Strain 0 Pokérus glitch (Generation II/Ruby and Sapphire, Generation II version)

Explained here.
bulbapedia.bul...
In Generation II, due to a glitch there is a 15/255 chance Pokérus (already has a ~1/21,845 chance of being applied to a party Pokémon after battle) is the "strain 0". Strain 0 Pokérus goes away but doesn't apply the dot next to the party Pokémon's HP, and it's possible for that Pokémon to get Pokérus again. This is unlike other forms of Pokérus which give the dot and make that Pokémon immune to Pokérus again.
Strain 0 Pokémon uses the left nybble of the Pokérus status byte as 0, while the right nybble is the number of days left until Pokérus is cured. So once a day passes and it decrements from 01 to 00, the game considers that Pokémon as never been infected. Say the status nybble was another value greater than hex:0F take 21; it would decrement to 20 (1 days left to 0 days left) but the game would still be able to use the left nybble 2 to track that the Pokémon had Pokérus in the past.
There is also a strain 0 Pokérus that can be generated by the RNG in Ruby/Sapphire (I might cover it in another video). Beginning from Emerald, it can no longer be generated by the RNG.
In this video I also show how you have to close the Pokémon menu for the Pokérus status to be removed, even if the day rolls over to midnight while the menu is still open but you checked the party in between.

[@muma-kitty1639]

this would explain why my pokemon in crystal seem to have no signs of pokerus infection after their contagious phase ended. good thing i saved a seaking in stadium 2