Bursting with parasites  You know things are going badly when
two thirds of your body cavity is filled with parasitic
bacteria. This is the unfortunate fate of a creature called a
Paramecium (Paramecium caudatum) when infected by a nasty
bacterial parasite called Holospora undulata.  For my current
research project I’m working with these species to study
evolution in the context of disease epidemiology (more on that
another time) and as such I spend a considerable portion of my
day staring down the microscope at these little critters.
Healthy Paramecium / Antonio Guillén / CC BY-NC-SA 2.0  A
Healthy Paramecium / Antonio Guillén / CC BY-NC-SA 2.0  Here is
a normal healthy Paramecium seen under the microscope. Each
individual is just a single cell, albeit quite a big one on the
cellular scale – about 0.17 to 0.29 mm – just about visible if
you hold a tube of them up to the light. For comparison, we
humans are each made up of about 10 trillion cells.  Paramecia
live in freshwater and eat bacteria, tiny algae an