The problems related to successful treatment modalities, and remission of cancer in general is many fold.
For starters, many people aren't aware of these issues at times until it's already too late. Many cancer related findings are completely incidental in nature. The truth of the matter is if it's progressed to a certain point, there isn't much that can be done other than buying some time.
Another factor to look at is the complexity of the immune system response. Our bodies immune system is designed to destroy foreign antigens/bacteria/viruses, however it's not as cut and dry as it seems.
The problem with many cancer treatments is this. Some cancer cells are completely undetected by our immune system, and are allowed to proliferate very precipitantly in some instances. It is not known why this happens, but many new treatments are geared towards helping the immune system recognize these cells, and destroy them early on.
Cancer also has a primary site, and can metastasize to other parts of the body. This complicates an effective treatment modality even further. At this point, there is new research and studies that have developed with some promising new treatment methods, but as far as a cure all, it just doesn't exist yet, and very well may never.
Now here's another example in regard to effective treatments of a viral component of terminal illness such as HIV. The reason why HIV is unable to be eradicated from the body is for a variety of reasons, many of which make a vaccine or cure an almost impossible task at this point in modern medicine.
Hiv is known as a retrovirus. It primarily attacks CD4 lymphocyte cells that are responsible for organzing an effective immune response (It also infects many other cells in the central nervous system and GI tract as well) by injecting a reverse transcriptase component into the CD4 cell. This basically invades the cells DNA structure and programs it to make replications of the HIV virus. Eventually, that cell bursts and hundreds of thousands of new HIV viral particles are released.
Now, the reason this is so hard to treat is complicated. The HIV virus primarily resides within the lymphatic system (or where our immune system is based from in attempt to limit systemic involvement.) The HIV virus also has some circulating in your blood, as well as your nervous system and GI tract. It's very good about once outside the lymphatic system in hiding from the immune system. This is primarily why your body can't fight it off completely, as it isn't able to detect all the viral particles that exist and destroy them. Another difficult compnent is the bodies ability to recognize already infected cells to destroy them before more viral particles are released.
HIV also has an uncanny ability to mutate, and form resistance to antiviral/protease inhibitor treatments. In other words, eventually with our current treatments available, the virus is able to alter its protein coat antigen rendering antiviral medications useless. Now this takes a long time, but it can be precipitated if people aren't compliant with their medication regimen.
Our immune system puts up a valiant struggle to control it, and it does for a long time in most (on average 10 years before aids criteria is met), but eventually the virus is able to overtake the immune systems response. The result is the inablity of your body to form an effective immune response to foreign invasion, and you succomb to opportunistic infections. HIV doesn't kill you per se, the opportunitic infections that are allowed to proliferate do resulting from an ineffective organized immune response.
I still haven't read this article completely yet, so i'll go do that now.
/rant :lol: