Infection
An Infection is a status effect inflicted by certain zombie bites that are particularly deep. If untreated, the infection will eventually kill you. You can clean the wound with antiseptic, or take antibiotics if the infection progresses. Always keep at least one infection treatment when you intend to fight zombies, one bite can be fatal.
When wearing filthy clothing, every incoming attack has a (very small) chance to turn into an infected attack causing a bite wound.
This is not the same as a traditional zombie movie infection. While its chance may - or may not - be increased by the zombie 'virus' (not a virus, but that is a different spoiler), it is most likely from their semi-rotting, foul-smelling saliva, combined with the human flesh still stuck between their teeth. The combination of bacteria from these sources eventually causes some form of sepsis.
Bite wound
Zombie bites that hit and deal any amount of damage can leave bite wounds that feel "really deep...". Such a wound can easily become a serious threat; though you've got about 6 hours after being bitten, you should pay attention and take care of the problem ASAP as it gets tougher to treat over time. An bodypart with a bite wound is blue in the interface.
For the first two hours you'll occasionally be warned about it ("Your bite wound really hurts.").
After 2 hours pass, the wound will cause you pain (up to 10 points), making sleeping very difficult ("Your bite wound feels swollen and painful.") and reducing your dexterity by one.
After 4 hours the penalties will double - two points of dexterity and up to 20 points of pain.
After 6 hours untreated, the bite wound will become infected.
A healthy diet, along with relevant traits such as Infection Resistant, can help your system repel the infection on its own. The chance of this happening to new survivors without Infection Resistant is about ~30%.
You can clean a bite wound with antiseptic, or painfully cauterize it with a hotplate or soldering iron. It should be noted that cauterization's success is not guaranteed (50% chance), and if it fails may even make the wound worse (as if 260 minutes of infection has passed, or 4.33 hours). If you have disinfectant, use it. Once the infection's fully developed, cauterizing or cleaning won't help as it's already spread into your body. You can also cauterize wounds with a knife if you've got fire nearby (an adequately filled lighter will do).
Getting bitten in the infected body part makes the infection progress slightly faster (~7 minutes per bite if bite wound or ~4 minutes if infected wound).
Searching the bathrooms of homes is a good way to find disinfectant or hydrogen peroxide - both of which can clean a bite wound.
Infected wound
Infections may develop when a bite wound isn't addressed within 6 hours. This disease will last for 24 hours. An infected bodypart is green in the interface, and shows up in the @ player info screen as 'Infected (body part)'.
After getting this disease (an Infected wound), your strength and dexterity will be reduced by one point each and you'll start accumulating significant pain (up to 20 points). The constant pain makes sleeping nearly impossible. You'll also have a low chance (0.1% chance per turn) of vomiting. Since this stage of infection is still relatively mild, one has a better chance to try and find medication and healthy food to try and fight the infection.
After 8 hours (Badly Infected wound), your strength and dexterity will decrease by another point each. You'll also suffer from severe pain (up to 60 points). The chance of vomiting increases significantly (1 in 600 chance). This stage of infection makes it more difficult to loot any place in, but if one has certain medication to increase their stats at least temporarily, they may still have a chance to find something before they fall severely ill by stage 3 infection.
After 16 hours (Pus Filled wound), your strength and dexterity will be reduced by yet another point each. You'll also occasionally vomit, will get fatigued frequently and suffer from very severe pain (up to 90 points). Vomiting becomes frequent (1 in 200). Worst of all, once the player reaches this stage of infection, they will become instantly exhausted no matter how much sleep they had the night before, and fall asleep quickly, this can lead to the player being very vulnerable in this coma, and if they do by a slim chance wake up, they are likely to end losing consciousness again soon afterwards. Due to the severity of this stage of infection, one is unable to move very far and will practically end up bedridden, therefore, one can only try work with what resources they have as they hope to recover from the illness by themselves.
24 hours after the infection (30 hours including the bite wound) you will die ("You succumb to the infection.").
You can cure the infected wound by taking one dose of any of the antibiotic variants -- destragon (strongest), regular antibiotics, or Atreyupan (weak). This requires taking one dose every 12 hours. Even with antibiotics, you may still fail to fight off the infection and die. If you succeed, you will be given the "recovering" status ("Recovering From Infection") with duration equal to 4 times the time spent with the infected wound.
Autodocs, automated medical devices that can be found in some locations, can administer destragon.
An infection can randomly heal on its own. A starting survivor has roughly a 15% chance of surviving an untreated infection. This chance is significantly higher with the Infection Resistant trait, which is alone as potent as being under the effect of regular antibiotics and stacks with antibiotics.
This disease can also be part of an NPC mission, where you'll be asked to bring antibiotics back.
Recovering from infection
After healing infection, the penalties are not removed immediately. It will take 4 times as long for the penalties to be removed as it took for them to accumulate. Penalties during recovery are the same as penalties during the infection, except they decrease with time rather than increasing.
For example, a survivor who healed a 10 hour infected wound (Badly Infected) will keep the Badly Infected penalties for 8 hours (4 * 2 hours), then they'll decrease to Infected penalties for the next 24 hours (4 * 8 hours).
Recovering from infection status significantly lowers the chance of other bite wounds and infections healing on their own.
If one was at the most severe stage of infection and starts recovering, it is wise to still avoid any towns until the infection has either recovered completely or to the least severe stage of the illness.