An Evolution Egg outbreak is a disease that happens in eggs. These only seem to occur in the Hidden Country.
What is the disease? The disease happens in eggs that are a pokemon who can evolve only. Within a week of being infected the pokemon will start to evolve in the egg. How can you tell if the egg is infected? You can't, which makes outbreaks more difficult to tell when they happen until an egg has hatched. Before they start to form they can be treated which will make it so that the pokemon doesn't evolve. You can't still treat infected eggs while they're forming but it will stop the process leaving a half-evolved pokemon, or a Mutated Evolution Egg. When this happens the pokemon normally can't evolve further if they're able. What would happen to an Eevee? They'd evolve into any of their evolutions, so it'd be a surprise.
What are the side effects? Common side effects are differently coloured eyes, but this doesn't happen all the time. A Mutated Evolution egg also tend to be disabled in some way. Uncommon side effects are that pokemon can't learn any moves after the age of 10, this is a rare side-effect for Mutated Evolution Eggs. Rare side effects are that a pokemon could turn shiny or half shiny, half shiny is more common in Mutated Evolution Eggs. As mentioned before unable to learn moves after the age of 10 are rare for Mutated Evolution Eggs.
Can they reproduce? If so are the offspring normal? They can reproduce, and the offpsring are completely normal pokemon even though the genes of one parent, or even both, are messed up.
How many outbreaks have there been? So far only 2 recorded outbreaks have happened. The first one occurred in 2007 in the Hidden Country, the second in 2016.
- The only current known/important Evolution Eggs are Pix, a Vulpix/Ninetales Mutated Evolution Egg whom has a few shiny bits on her, that's her only side effect. The other is Rita who is a Gothorita, she seems to have no side effects at all.
- The second outbreak caused an underground criminal organization to go after Evolution Eggs for a cure to make them evolve, which is strange for a bunch of criminals. The only successful capture was Rita. They never found a cure with her before she escaped.