There are six species that account for 95% of all detected mycoplasma contaminations in continuous cell line cultures: M. orale, M. arginini, M. fermentans, M. hyorhinis, M. hominis, and A. laidlawii1. Typical routes of contamination are: cross-contamination from other cells (for example, via aerosols generated during pipetting), using the same media bottles, or handling more than one cell type at a time.
Other sources of contamination within your cell cultures could include direct contamination from the researcher, as well as the use of contaminated materials, such as animal sera. The high risk of contamination highlights the importance of always purchasing high-quality cell culture media from reputable manufacturers.
Reference:
1) Drexler HG, Uphof CC (2002). In Vitro Cell. Dev. Biol. – Animal 38:79–85