THE MICROBE 
19 
thousand times, will not show them. They are 
known only by the work they do. They are 
called either “ultra-microscopic,” because they 
cannot be seen with a microscope, or “filter 
passers,” because they pass through the finest 
filters we can make. The germs which cause 
smallpox, measles, infantile paralysis, or rabies, 
are all filter-passers, and have never been seen 
under a microscope. 
They are the simplest as well as the smallest 
living things. Those that are large enough to 
be seen under the microscope have been found 
to consist of only one cell, instead of many cells, 
as do larger plants and animals. Though they 
have no green coloring, most of them belong 
to the plant kingdom. The different kinds of 
microbes have different shapes, and the shape 
of a single kind of microbe often changes as it 
grows. The smaller microbes may be roughly 
divided into three simple kinds, depending on 
their shape: the round kind, called the coccus; 
the rod-shaped kind, called the bacillus; the 
spiral-shaped kind, called the spirillum . 
Are there 
microbes we 
cannot see? 
Spirillum 
Coccus 
Baccilus 
