Mycoplasma contamination of cell cultures comes from lab personnel or from contaminated culture media.  Mycoplasma are too small to see with a conventional microscope and don’t change the pH of media or cause turbidity, but they can cause serious cellular changes and can ultimately destroy a cell line.  The most common signs of Mycoplasma contamination are poor cell growth and/or death of a cell line after just a few days in culture.  Working with a Mycoplasma-contaminated cell line poses great risk to other cell lines in a laboratory, so identifying a contaminated cell line is critical.