Click on the build praefecture button. Instead of left clicking on a grass 
tile to build a praefecture, right click on a tile occupied by a house to 
infect that house with disease.

If your farms and/or trading posts are doing very well or there is more 
than one product coming into them then you can add more warehouses to store 
surplus or other types of products.

Press 'A' on City or Province mode to accelerate time.

Mondo Money:
While playing, go into your forum and exit. Once outside, choose a building, 
then click on a blank section of the menu bar. Keep holding the mouse button 
drag your cursor to wherever you wish to build. BE CAREFUL!
After dragging away from the menu bar, every space your mouse pointer crosses 
will be filled with the
building you chose. Note that glitches can occur if you do this. For example, 
the circus and circus maximus become halved if the space given can only hold 
half. Also a decrease in money might happen if you try to leave the screen to 
enter another. To prevent this, just build something that takes money away. 
If you build something with money then do the trick, it will take away that 
object and give you your money back. This trick allows you to get big bucks.