This is a major concern for every character. There are various diseases (not only sexually transmitted diseases) in this adult game and they basically break down in two categories: Airborne and sexually transmitted. The first type of disease is caught just by being close to an infected character and is therefore very insidiously. The second sort of disease can only be transmitted during a sexual intercourse, which is of course what you will be doing alot in a sex game. Diseases also have various degrees of contagiousness. Some may have only a slight chance of being transmitted, others may spread like a wildfire.
All diseases are programmed individually and can show different symptoms and effects. All diseases go through different stages and each stage can have a different effect. Some diseases just heal when the end stage is reached, some remain at that stage and some eliminate your character. Some diseases can be determined by just looking at the character since it may show some signs like spots or ulcers, others can only determined by a medical biocorder (a diagnosis device) or other means. Don't let a Diarrhea ruin your adult game fun for the day! Get some medicine.
Healing a disease can be tricky. The only possible cure for a disease is a specific drug that is tailored to this disease. The adult game system provides 10 chemicals as raw materials and these chemicals must be mixed in a specific formula to target a disease. What makes things worse is, that some cures may only heal a disease in a certain stage. It may reduce the stage to the preceding one or it may cure the disease completely.
Finding these formulas can be a VERY challenging task. Recipes must be collected in recipe books by pharmacists which are precious goods because they can not be shared. Having a recipe for a certain cure can be a gold mine, depending on how common this recipe is.
These are serious features that often aren't even in a non sex
Drugs can be taken by the ill characters, but some drugs may require a doctor with medicine to administer them in order to prevent unwanted side effects. Side effects can alter your abilities or skills permanently and are generally a bad thing, so it's a good idea to go to a doctor instead of just shooting them up your arm yourself. However you can do so if you want to, after all in an adult game you make your own decisioins. There are also some drugs that alter your abilities temporarily, for example give a STR boost. However, all drugs also have a chance of creating a drug addiction!
If your character becomes a drug addict, then one ability is lowered dramatically for a certain time, for example STR or INT, but there night also be other negative effects that affect your adult game play. Addictions can be countered in two ways: You may ignore them and just wait about one week after which you are cured, or you may decide to fight the negative effects by using drugs that are specifically tailored to those addictions and which suppress the negative side effects for one day. However, those drugs are illegal in London and you must find a pharmacist willing to create those drugs or a supplier who takes the risk of selling them. Whenever such an illegal drug is traded, there is a chance that a crime is generated either against the seller but more likely against the customer, so this business is very risky and dirty.
Searching for cure will be one major part of the game, since diseases are everywhere! Consider working together with other characters on this task! Perhaps you found and run a hospital?