There is a lot of wild speculation about this on the internet.
From my experience, I believe the mice always appear regardless of what you do, but they have no effect on anything (patients don't complain, health inspectors doesn't care etc.).
The only thing you can do it kill them with your cursor, which creates mess that has to be cleaned up and you can ...
I think those failure conditions are just random headlines that doesn't mention why you lost.
If you click on this button in the lower right corner:
then you get to a "Winning condition" screen, where you and 3 other hospitals are trying to win. The first hospital who reached 100% in both people cured, money, and hospital rating will win the scenario.
The only downside I can see is the extra bit of time it takes to place them but I guess that it's time well spent.
Furthermore according to this strategy wiki adding windows should also make it easier to place and edit furniture in the room.
The game is lost when, at the end of the year, you have any goal below an established threshold. The headlines are random, so you'll have to check your progress report status before you lose.
Below a certain threshold, the bar will turn red and will grow as you get farther from the goal. If it is at 100% at the end of the year, you will lose the game.
What I've found on the web, so far:
Fire extinguishers may seem useless, but they do the same job as plants and don't need watering. They also delay the explosions of machines, therefore they are more useful in rooms.
Go to your Research Department window (the one with the tubes), and increase the bar of "Improvements"...
I've done some preliminary investigative work, and in reality, it's not actually quite so simple: there's no 'set' price everyone will pay: each patient seems to have its own payment tolerance.
It appears true that even as low as 105%, some patients will refuse to pay at the base reputation level. At 145% it appears no one will allow treatment anymore, ...
Theme Hospital is a DOS game (well, and playstation) and as such only runs on VGA resolution.
There are open source clones which support higher resolutions though I haven't tried them, for more information on that see https://www.gogwiki.com/index.php/Theme_Hospital
As a bonus of the open source clone you don't need a virtual machine anymore, ...
More doctors - yes! Each active researcher in the Research Department (i.e. a doctor who is able to sit at a free desk and get in and out of the room) will reduce the amount of time required for developments to be researched.
Additional research departments, however, are unlikely to provide a notable benefit. The only thing you'll get from two research ...