Mycoplasmas are the "nanobots" of the germ world. They are the smallest of all pathogens and are distinguished by the fact that they have no true outer shell. These two facts allow it to go into "stealth mode" and hide deep within tissues. In fact, mycoplasma has been know to hide within cells. Tricky, eh?
Unfortunately, mycoplasmas are not only elusive but also deadly and everyone should know a little about them, because, quite often, medical professionals are not caught up with the latest research. First of all, there are many variants of mycoplasma, including the infamous Mycoplasma Pneumonia, better known as "walking pneumonia." Of course, walking pneumonia manifests itself in an obvious fashion with its classic fever and cough symptoms. Another example of a more "obvious" mycoplasma is mycoplasma genitalium, a common STD that causes many of the same symptoms as chlamydia and gonorrhea.
However, all of the mycoplasmas (and some others) have a darker side: they can linger in tissues for years or decades, undetected and unnoticed. The medical profession is just beginning to realize what these sneaky and hard-to-eradicate pathogens do to the human body.
Look at some of the nasty conditions and illnesses associated with mycoplasmas and already verified in the literature:
1. Prostate Cancer. One 2007 study showed that M. hyorhinus likely contributed to prostate cancer  and an ensuing study two years later verified this and showed the same for M. genitalium.  This is significant, becuase M. genitalium is the kind of mycoplasma that causes the fairly common STD.
2. Cancer. Early animal studies showed that mycoplasma could lead to cancer  and epidemiological studies on humans verified the same. 
3. Arthritis. Many bacteria can cause "reactive arthritis," i.e. an outbreak of arthritis that results in the body's response (inflammation, etc.) to a pathogen. Mycoplasma is no exception and the "walking pneumonia" variety has been known to trigger reactive arthritis in some cases.  A recent study showed that the genital, STD-related variants did the same thing. 
4. Low Testosterone. Some researchers feel that mycoplasma infections can lead to low testosterone by either altering the hypothalamic-pituitary axis  or by consuming cholesterol itself.
5. Male Infertility. Mycoplasmas attach themselves to sperm and wreak havoc for the little swimmers. Many studies show that mycoplasma leads to male infertility through decreasing sperm motility and invasion of the testes.   
Mycoplasmas have been suspected of causing many more issues and conditions and research will undoubtedly verify many of these over the next decade. The good news is that most of these can eradicated through antibiotics, but the trick is that both you and your doctor must be informed as to their pervasiveness in order to initiate testing and treatment.
A strong Immune System is absolutely critical as well. Some of the variants cannot actually be completely killed by standard antibiotic treatments but are just rendered immobile and/or inactive. The body must put the final "dagger through the heart". of these microscopic critters.
Another reason that keeping your Immune System strong and healthy when it comes to mycoplasma is that many experts have noticed that mycoplasma is often held in check within the body, but when a immune-injuring stressor hits, an outbreak of mycoplasma is initiated. Again, read my link on How to Prime Your Immune System for ways to help protect yourself.
1) Cytogenet Genome Res, 2007, 118:204-213, "Exogenous mycoplasmal p37 protein alters gene expression, growth and morphology of prostate cancer cells"
2) PLoS One, 2009, 4(9):e6872, "Persistent Exposure to Mycoplasma Induces Malignant Transformation of Human Prostate Cells"
3) PNAS October 24, 1995 vol. 92 no. 22 10197-10201, "Mycoplasmas and oncogenesis: persistent infection and multistage malignant transformation"
4) Canadian Journal of Microbiology, 2001, 47(8):691-697, "Do mycoplasmas cause human cancer?"
6) Scandanavian Journal of Rheumatology, 2006, 35(6)459-462, "Potential triggering infections of reactive arthritis"
7) NeuroReport, Apr 19 1995, 6(6), "Mycoplasma fermentans activates the hypothalamo-pituitary adrenal axis in the rat"
8) Fertility, 1984, 7(1-4)193-202 (16 ref.), "Mycoplasma infestation of the testes as a cause for infertility"
9) Andrologia, Oct-Dec 1975, 7(4):309-316, "The Correlation of Human Male Infertility with the Presence of Mycoplasma T-Strains"
10) Fertil Steril, 1975 Dec, 26(12):1212-8, "T-mycoplasmas and human infertility: correlation of infection with alterations in seminal parameters"