Infections are a core mechanic of Flexible Survival. As you try to survive the apocalypse, you will inevitably be transformed by the nanites that were released upon the city.
An almost inevitable result of playing the game is infection. In most scenarios, the player starts as a human, but so did the majority of the other creatures the player will encounter in the city. Almost none of the city's occupants remain full blood humans. If the player is careful, they may remain human for the duration of the game, but much of the fun revolves around the various infections the player encounters and may acquire. Any given infection will transform the player's physical and sexual attributes toward that infection's natural shape.
Generally speaking, there are two ways in which players become infected. The first is through sexual interaction with certain Monsters (generally in a non-consensual fashion) or NPCs (generally more consensual, though just as sexual). The player may also acquire various Items through the course of the game which bestow infections upon consumption or use.
When infected, a random part of the character's body will be subject to infection and change. For the purposes of the game, there are five parts of the body which can become infected:
There will then be a description of the change occurring to the character's body and their personal description will be altered to reflect this change. Further infection from that same strain will affect a new body part each time until the player's form is fully transformed, leaving only any remaining shifts in gender to continue. Some NPCs and events can result in infection as well, as a player makes contact with a tainted object or infected individual. Some NPCs do not infect the player or only do so under special circumstances. Some basic feats, such as Microwaved, Resistant and Mutable affect a player's likelihood to become infected, but generally don't affect non-random infection.
Gender shifting: Most creatures and infections have a target gender they want to move the player towards, with assigned sizes and numbers for cocks, cunts and breasts they seek to bestow. A gender shift will incrementally move the player towards these values. Changes to the player's body will also prompt a shift in the player's gender and breast size, if applicable. An infection affecting a player's cock will change the form of their cock as well as causing a gender shift.
Once the player has become fully infect by a strain (all 5 body parts), gender and breast changes will continue until the player's sexual changes are also complete. This completed status occurs when the player's genitals are at or between the creature's listed value and 150% of the listed value. If one of the player's sexual characteristics falls within this range, it will not be changed by the infection attempt, though any outside the ranges will still change.
Players are able to exert some control over the type of gender shifting that can take place, should they choose one of the various 'Gender Preferred' or other fun feats related to gender shifting.