Cancer

No misleading titles or introduction this time, folks. Cancer doesn't require a fanciful introduction, or a surprising twist to make it scary. Cancer is one of the most scary words known to the English language, especially when spoken by a doctor. In literary terms, something is termed as 'cancerous' if it spreads to and withers or destroys everything it touches. Most of us know somebody who has either been affected by or killed by it, and nobody doubts its ability to kill or harm. Just so we're talking real numbers though, the CDC vital statistics estimates that a little over 574,000 people died of cancer in the United States in 2010. [1]

So what is a cancer, exactly? To put it in layman's terms, cancer is a cell that, through mutations accumulated by any number of ways, has forgotten how to die, and often grows unregulated. When deprived of growth medium, or become damaged, most cells will commit suicide through a process known as apoptosis. Cancer cells, however, will survive inappropriately. Some cell lines even become 'immortal' cell lines, such as the Henrietta Lacks or 'HeLa' cervical cancer cells that are still in use today, over fifty years after the donor died. 

There's a few reasons cancers are hard to treat, but one of the biggest problems is that when the cell turns cancerous, it continues to express the right Major Histocompatability Complex (MHC) molecules- the ones that identify you as you to your immune system- on the cell surface. Even though the cancer is growing and spreading rapidly, your immune system won't attack it because it sees the cancer as you. The good news is that, because humans have a wide MHC variability, it's very difficult to transmit or transplant a cancer from person to person. What would happen, though, if we had very low MHC variability? 

Devil Facial Tumor Disease (DFTD) is a transmissible cancer among Tasmanian Devils. The animals bite each other on the face, spreading cancer from animal to animal through the bites. Initially thought to have been the result of a tumorogenic virus, investigators have determined that it's actually the cancer cells being transmitted. The cancer is both disfiguring and lethal to the Tasmanian devils. 

Another big name in this field is canine transmissible venereal tumor, or CTVT.[3] CTVT is a transmissible cancer spread among dogs by sexual contact. It's not typically fatal, but recent genetic studies estimate the tumor is 11,000 years old, having persisted by jumping from dog to dog. If you believe Wikipedia, there's a third transmissible cancer spread among rodents by Mosquitos. 

Humans aren't at a very high risk for transmissible tumors because we have widely variable MHCs, but that doesn't mean it's impossible. If, for example, a tumor developed with a widely permissive MHC. What we do have are retroviruses that cause cancer by inserting their genomes into ours- such as Human Pappilomavirus. It's not that the virus' goal is to induce a tumor, so much as gene promotor or enhancer sites get accidentally copied and inserted into the host genome again. 

What makes cancer scary is that it's not some virus hijacking your protein synthesis machinery, not a bacteria or a parasite feeding on you, it's you. Sometimes, though, maybe it's someone else. 

[3]http://m.petmd.com/dog/conditions/cancer/c_dg_transmissible_venereal_tumor

[2]http://www.dpiw.tas.gov.au/inter.nsf/WebPages/LBUN-5QF86G?open

[1]http://www.cdc.gov/nchs/fastats/deaths.htm