Pokérus - spreading to newly hatched Pokemon?

[PhantomUnderYourDesk]

Thanks a lot, Iron, for your detailed reply.

I overlooked beneficial microorganisms in this case, sorry for that flaw. Plus, I was aware that viruses  like HIV can indeed be spreaded to an offspring, if at least one of the parents is infected - but I excluded them because viruses are no "living beings", and it's highly questionable whether they can be considered as "microorganisms" as well. After all, they have an own branch called "virology", same on the tree of life.

And I honestly did not know that eggs may catch Pokerus, hence thanks for the information. Through the membrane of the eggshell, you say? Hmm... just found an old but interesting looking scientific review online on how that may work on poultry eggs. To avoid copyright issues, I restrain from sharing the link to it, though.

(...)

Infection
When a Pokémon is infected with Pokérus, their status screen will display an icon indicating this special "status" the same way status conditions would be noted. If the Pokémon becomes affected by a status condition such as Sleep, the Pokérus icon will be temporarily replaced until the status condition is cured. As long as the Pokémon is infected with Pokérus, it can spread the virus to other Pokémon in the player's party.

Pokérus may spread if an infected Pokémon is in the player's party after a battle. Pokérus may only spread to a Pokémon directly adjacent to an already infected Pokémon and only if they have never had Pokérus before. From Generation III onward, Eggs may catch Pokérus like any other Pokémon.

So it is not inherited from the mother, but passed through the eggshell membrane from other infected party members.
And you don't have to participate in a battle to gain it, but your party needs to battle for you.

What goes against this though, is that I don't remember battleing anything/anyone during hatching that particular Minicorn. In fact, it hatched during taking a break from cycling around Route 9 and buying some supplies in Bealbeach's DepartmentStore... look at my newly hatched unicorn, Dad!