The smallest of the pathogenic or disease-producing bacteria is the influenza bacillus, 1/51000 of an inch in length and 1/102000 of an inch in thickness; and among the largest is a bacillus causing an animal disease which is 1/2000 of an inch in length and 1/25000 of an inch in diameter.

Among the free-living non-pathogenic forms much larger examples are found.

In shape bacteria are round, or rod-shaped, or spiral; the round forms are called micrococci, the rod-shaped bacilli and the spiral forms are called spirilli.

A clearer idea of the size is possibly given by the calculation that a drop of water would contain one billion micrococci of the usual size.

Their structure in a general way conforms with that of other cells.