game with the conceit that you are a swarm of medical nanobots injected the patient as an experimental last-ditch effort to save their life. you can expand your swarm and create all sorts of specialised bots, but all the resources to do so are ultimately from the patient's body, so if you drain them too much they die. you are fighting against tumour cells, though, who have no such concern, so maybe sometimes taking too many resources for yourself is better than giving it to the cancer?
envisioning the gameplay as divided in stages, the first one being just 'get rid of all the cancer cells before the patient dies', where you have to balance aggressive expansion to improve your capabilties with not throwing the patient's vitals too out of whack, researching better 'weapons' that kill cancer cells faster or with fewer deleterious health effects, etc,
then a 'solidifying' phase where the cancer is gone, but the patient might be in poor health and you start building systems to catch opportunistic infections, automated systems that respond when something about the patient is off (fever, low blood pressure, broken arm, whatever), and some rudimentary communications capability. not enough of whatever mineral in your resources? you could communicate with the little wearable medical device the patient has on their wrist to tell them to change their diet! or, y'know, you could give them a craving for a specific kind of food. either works
and then in the third stage, you really go deep into host manipulation and host cooperation mechanics. you can make your host superhuman, get messages from them about what changes they want to their body and implement them, and in return they do things for you. change their diet, buy a better communications interface, tell all their friends that medical nanonbots are great. or you can effectively turn them into a zombie, just a vessel for the bots. or split the difference, and make the patient want specific things that the swarm can easily provide to build 'trust'.
yes this does mean you can induce your host to want to be a girl and then be really happy with you for speedrunning their medical transition
