40d:Miasma

Miasma is a "noxious odor" or otherwise-poisoned air, believed to spread disease. The theory of miasma as the general cause of disease propagation was quite widespread prior to the 19th century, and although in some respects it is correct that some diseases may be transmitted via the air, not all diseases are transmitted by such mediums.

In Dwarf Fortress, the only impact a miasma will have is to disgust dwarves encountering it, giving them bad thoughts. However, one miasma can disgust a large number of dwarves, especially if it is in a main corridor.

The most common cause of in-game miasma are rotting corpses or body parts left inside. Often the corpses of small vermin killed by cats will cause a miasma. You can limit your dwarves' exposure to miasma by setting up a refuse pile (outdoors, or in an enclosed space -- see refuse). Rotting items left outside will not generate miasma.

Miasma will not travel through walls, ceilings or doors.

It also does not "spread" diagonally.