Except in some scenarios, players can insert one of the genes from Genetic Code before they start their game. These genes allow personalization of plague, for example, by providing boosts to infectivity and DNA gain, increase/decrease the mutation rate, and so on.
Every time a player wins a game with a standard plague, they unlock one of the 25 genes regardless of the difficulty level. The same goes for Neurax Worm. However, the other special plagues can unlock one of their special genes.
There are five different types of genes: DNA Genes, Travel Genes, Evolution Genes, Mutation Genes, and Environment Genes. Players can only have one of each type of gene per game, meaning that there are a large number of different genetic combinations that one can use for any disease (3,125 to be exact). There are exclusive genes for Necroa Virus, Simian Flu, and Shadow Plague.
On Android, you must buy the premium version for $0.99 in order to use the genes that you unlocked.
Ability costs don't increase but easier to cure (-0.1 infectivity in all environments, -0.11 cure requirement)
Note: In the Mega-Brutal difficulty mode, even with the "stasis" genes, the cost to evolve still increases as more people are infected, but with a lesser extent than without the "stasis" genes.
Special Plague Genes
Necroa Virus Genes
Exclusive to Necroa Virus, replacing Evolution Genes
You can unlock genes by playing the game in the Casual difficulty.
Beating any plague type unlocks a random gene.
Beating a Necroa Virus game unlocks a random Necroa gene.
Beating a Simian Flu game unlocks a random Simian / Artificial gene.
Beating a Shadow Plague game unlocks random a Shadow / Flight / Blood Rage gene.
Multiplayer Genes
Here is a list of some genes in multiplayer games. Some of the genes in multiplayer mode are exclusive to VS Mode.
Hydrophobe: Gives your plague a bonus in arid climates, but a penalty in humid climates (+1.33 arid, -1 humid)
Xerophobe: Gives your plague a bonus in humid climates, but a penalty in arid climates (-1.33 arid, +1 humid)
Urbophobe: Gives your plague a bonus in rural environments, but a penalty in urban environments (-0.67 urban, +0.57 rural)
Rurophobe: Gives your plague a bonus in urban environments, but a penalty in rural environments (+0.67 urban, -0.57 rural)
Pharmaphile: Gives your plague a bonus in wealthy countries, but a penalty in poor countries (+0.12 rich, -0.5 poor)
Pharmaphobe: Gives your plague a bonus in poor countries, but a penalty in wealthy countries (-0.12 rich, +0.5 poor)
Frigophobe: Gives your plague a bonus in hot countries, but a penalty in cold countries (+0.1 hot, -0.16 cold)
Thermophobe: Gives your plague a bonus in cold countries, but a penalty in hot countries (-0.1 hot, +0.16 cold)
Benign infection: Your plague is harder to cure, but you start with less DNA
Unstable inception: Get bonus DNA at the beginning, but your starting country will be selected randomly (+9 DNA at the start)
Safe house (VS mode exclusive): Get more DNA (from 2 to 4) from bubbles in your starting country, but your opponent gets bonus DNA (+6) when it is discovered
Search and Destroy (VS mode exclusive): Get a DNA bonus from finding your opponent's starting country. The bonus decreases the longer it takes
If you find your opponent's starting country in 0 days, the bonus will be 40 DNA points. The bonus will be decreasing by 0.13 per day.
Base Hydration: Your plague can mutate transmissions and abilities, but costs to evolve will increase quickly
Peekaboo (VS mode exclusive): See which countries your opponent has infected when you find their starting country, but get less DNA (-6) when you find it
Innocent Infection: Your Plague will find it easier (+50%) to infect people that are already infected by other diseases, but harder (-50%) to kill them
Deadly Disinfectant: Your Plague will find it easier (+50%) to kill people that are infected by other diseases, but harder (-50%) to infect them
Trivia
Some of the genes have good and bad sides. For some genes that make your plague easier to cure, however, some abilities and symptoms can override that effect.
Neurax Worm is the only special plague that doesn't have its own special genes.