Unselectable Units?

I'm wondering - how do you create an unselectable unit?

For example, the Plague Cloud that comes from a Meat Wagon's attack is considered a unit, but it's normally unselectable. Even though Meat Wagons create tons of Plague Cloud units, you'll never be able to select one in-game. Moreover, the Plague Cloud is unattackable and has no collisions in game; it is treated like a graphical effect, even though it (strangely enough) is considered a unit.

My question is - how can I make a unit unselectable like a plague cloud? For example, say I wanted to make a unit look like fire, instead of using the fire doodad. Could I make that unit unselectable and unattackable?

You can probably achieve this by setting a unit's selection size to zero, collision size to zero, and then giving it the ability permenent invincibility. You should also probably remove both of its attacks and set its movement type to none. Other units will be able to walk through it, and with a selection size that small, a player shouldn't be able to manually select the unit (though it is still possible to select the unit using the trigger- Select unit).