There are seven normal plagues, each of which is unlocked by killing the world on Normal or Brutal mode with the previous plague. Bacteria is unlocked initially. The rest can also be purchased for $.99.
Bateria
"Most common cause of Plague. Unlimited potential."
This is the default plague. It is the easiest plague in the game to use, aside from Cheat Plagues, and has no downsides.
Virus
"A rapidly mutating pathogen which is extremely hard to control."
The Virus is a powerful plague, but it's unpredictable and will often cause random mutations that are hard to get rid of. These make it more noticeable, requiring players to spread it faster.
Fungus
"Fungal spores struggle to travel long distances without special effort."
The Fungus plague travels slower and with more difficulty than prior plagues. However, it has a few special abilities. Spore Burst and Spore Eruption both give it the ability to infect random, uninfected countries. Spore Hardening makes it much more resistant to climate changes.
Parasite
"Parasitic lifestyle prevents DNA alteration from every day infection."
Parasites are harder to detect than previous plagues. In addition, they also have a "Symbiosis" special ability which allows them to become even harder to detect. This allows the plague to travel quickly before people begin shutting things down.
Prion
"Slow, subtle and extremely complex pathogen hidden inside the brain."
Prion is a very slow plague. It takes a long time for people to notice or cure it, but it also takes a significanty long time to evolve compared to other plagues. It also has a Neural Atrophy ability which makes it even more difficult to find a cure.
Nano-Virus
"Out of control, microscopic machine with a built in kill switch."
This is a man-made weapon, composed of microscopic machines. Because it's manmade, governments will immediately know about it and try to shut it down (instead of curing it). Nano-Virus has multiple special abilities, all of which will make it more difficult to cure.
Bio-Weapon
"Exceptionally lethal pathogen that kills everything it touches."
Bio-Weapons are the only plagues that could be considered too lethal. As the game goes on, the plague becomes more and more deadly, making it hard to spread the plague before the host dies. Gene Compression makes it take longer to increase lethality, Nucleic Acid Neutralization temporarily resets lethality, Deactivated Modified Genes reduces lethality, and Armageddon Gene maximizes lethality and increases infectivity.
