So how do plagues work?

[Randallw]

I tested out the plague bomb on a homeworld and dicovered that it made a plague which had a minor effect reducing population at level 1, it was balance mod and plague 5 bomb only made level 1, which is still better than "makes level 0 plague" that was listed.

I checked some stuff like wiki and manual but I still don't quite understand its effect. It seems to slowly reduce population and apparently it reduces the planetary conditions so much population prefers to go elsewhere. I dropped it on a small planet with only 200M and they still grew, I assume by migration from homeworld.

Can someone explain its effects. Apart from making people flee the planet as they die off, does it make the facilities stop working or reduce efficiency as they are unhappy. Were the people dying or just migrating?