Very little, almost cell-sized anthros, living inside bodies called "cities". Each "city" has one "care taker that makes sure everything is ok and the host is healthy. The benefits are that the tiny anthros have shelter, and the host (who knows nothing about this), is a lil bit healthier thanks to the germ police.
Citisents travel from "city" to "city" with special buses-aeroplanes. They take electricity from their host's blood. They have long petruding towers in the blood vains that the blood cells rub against and create an electric charge (thanks to brother captain stern for this part)
Special scientists gather food from the host's stomach, process it in factories and sell it in the market. Because no hosts eats all kind of foods, care takers will export and import goods from other cities. The city's waste is carried and mixed with the host's own waste. The amount is so small that its not detected even by analysis. Special lights simulate night and day.
The care taker's main office is usually located in the head, where he can monitor everything. Normal citisents don't know they live inside a body.,