Paranoia is a tier 2 symptom found in all of the standard plague types. It makes people have irrational delusions. Their distrust of others makes them unlikely to seek treatment or cooperate with others.
On rare occasions, paranoia can cause a country to be so paranoid that it will remove a major part of its contribution to a cure.
Interestingly, when combined with Insanity, it can cause nuclear explosions, which are said to be sabotaged power plants. However, it's likely that this is just countries nuking each other.