The mushroom Coriolus versicolor (Turkey Tail Mushroom) is the most ubiquitous mushroom in the world. It grows on fallen trees and stumps from the sub-arctic to the tropics.
So considering these vastly different environments and all the other fungi and bacteria it has to compete with for survival, it's a qualified guess the Coriolus has a strong immune system, with an array of potential medicinal compounds.
In traditional Chinese medicine, Coriolus versicolor extract is used to treat liver cancer and some types of jaundice.  In modern medicine, the best known and most researched medicinal extract of Turkey Tail Mushroom is called PSK. It is used in Asia as an anti-cancer drug under the brand name Krestin. 
Two Japanese studies in the 1990's encompassing a total of 486 patients showed an increased survival rate from gastric cancer when PSK was added to conventional chemotherapy treatment. [161, 162, 163] It's also been found that PSK reduces cancer metastasis and recurrence. [161, 162, 164]
Two other compounds isolated from Coriolus have demonstrated an inhibitory effect on leukemia. A polysaccharide named CVP exhibited an inhibitory effect on leukemia cell growth with no harmful effect to normal lymphocytes. [165, 166, 167] The small polysaccharide SPCV also showed leukemia proliferation inhibition. 
It appears that Coriolus extract helps combat cancer in two ways:
1) By a direct inhibitory effect on the proliferation of cancer cells.
2) By enhancing Natural Killer (NK) cell activity. [169, 170, 171] Natural killer cells are a critical part of the human immune system's fight against cancer.
Efficacy against different types of cancers varies. PSK is used in Asia for cervical cancer as an adjunct to radiation therapy. It has been linked to increased survival rate from that type of cancer.  It was also shown to decrease tumor growth in hormone responsive prostate cancer by a study at New York Medical College in 2001. 
Other forms of cancer that PSK has been tested against with promising results include breast, lung and colon cancer. [25, 173] It appears ineffective against Sarcoma 180. 
PSK is also a powerful antibiotic, specifically against Listeria monocytogenes, Candida albicans, Pseudomonas aeruginosa, Escherichia coli, Cryptococcus neoformans and Staphylococcus aureus. [175, 176, 177, 178, 179]
Tests in vitro (test tubes / Petri dishes - not live subjects) have even shown a compound (PSP) from Coriolus versicolor extract to inhibit the replication  and infection of HIV. 
In a study conducted in 1995, PSK demonstrated effectiveness at normalizing the immune function of people with chronic rheumatoid arthritis. 
Paul Stamets, in his book Growing Gourmet and Medicinal Mushrooms, says he's aware of several individuals with Hepatitis C reporting symptom relief on a regimen of medicinal mushroom tea. In particular, he mentions a man with a swollen spleen and liver. Every day, he would drink a tea made from Ganoderma lucidum (Reishi) and Coriolus versicolor (Turkey Tail Mushroom). The swelling disappeared after two weeks. His liver enzymes normalized as HVC was reduced from 1.3 million to 140,000. 
Coriolus extract also is rich in antioxidants [184, 185, 186, 187] and has been shown to help the spleen recover after radiation therapy. 
Finally, in the book Mycelium Running, Paul Stamets lists two additional areas of promising research on Coriolus versicolor: Kidney health and uterine cancer. 
Note: The statements on this page have not been evaluated by the Food and Drug Administration. This article is not intended to diagnose, treat, cure or prevent any disease. Always consult a licensed medical practitioner before using any herb (or mushroom) for medicinal purposes.
Credits: Thank you, Paul Stamets, for research references.