Your trek in the outside world is... interesting. At the moment you comprise a mere 100,000 Nanocells, all configured for individual mobility. Each communicates through short range, high frequency radio that shouldn't interfere with other radio systems nearby, but means your individual Nanocells can't stray far apart. Thus you embark outside as a swarm of invisibly small dots, crawling over the ground fairly slowly. This is unfortunate, as your vision is a little limited and a pack of humans intersect you on the street, stepping on large numbers of your Nanocells. Luckily the surface of their shoes and the road you are moving on is very uneven at your scale, so very few of your parts are crushed, most fitting into the hollows and grooves as they fit together. Still you appreciate the need to get away from humans and their activities, or at least find some way to interact with them without them damaging you!
At first you crawl as far as you dare away from the laboratory where you were created, then seek the nearest building to get away from the walking humans above you. Your access to the scientist's computer gave you some data about the human world, but not enough. Also being so small, and the cameras on each individual unit being so poor at high definition vision, you cannot determine what the building is until you enter.
The interior however seems to be that of a hospital, where humans go to either be healed or to die. The 'TV shows' you have seen seem to indicate that a large number of humans who come as 'patients' die in such establishments. Perhaps a human that has just died would be useful to you? You may be able to get the body working again, and the human mind will no longer be present to claim ownership any more. Your swarm spreads out and locates two rooms, one with a young male and one with a young female, both seem to be in serious condition. Your limited understanding of human anatomy indicates they are both on the verge of dying.
You cannot decide so arbitrarily use a random factor to decide, and end up crawling your spread out mass into the small room of the young woman. Various machines are connected to her, some cleaning her blood, some monitoring her life signs. You alter your individual Nanocells structure, using triple groups of Nanocells to rework one other Nanocell, then moving on. Soon all your cells are coverted from mobility to interaction. You can still move them slowly, but mostly their motive systems work well in liquid, or by sliding through other objects. They also now use a combination of infrared, magnetic induction, and chemical triggers to communicate. Shorter range than before, but will work inside a human body.
After your trip here and the numerous times people stepped on you your swarm is down to a little over 700,000 units, but it is more than ample to the task of repairing this body. First though you must wait for the current occupant to expire.