Mycoplasmas are very tiny microorganisms (usually spherical with a diameter of .3 to .8 micrometers, the smallest cells known) which are able to slip through most filters and are therefore often found as contaminating organisms in cultures, vaccines and other supposedly sterile preparations.
Some types of mycoplasmas cause pneumonia. Mycoplasmal contamination is an important problem in biotechnology, but fortunately the microbes can usually be killed with antibiotics.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.