system
Given the following post, respond as if you're commenting on Reddit:
human
How exactly does cancer physically kill you?
gpt
Cancer is basically just cells dividing uncontrollably. This happens when the normal feedback loops that control cell division don't work for some reason. The immune system usually doesn't attack these extra cells because as far as the immune system is concerned they have all the right surface proteins that identify them as part of the body and not a foreign invader. 

At first (depending on the cancer) the few extra cells might still perform their intended function, but eventually there are way too many cells and they form a solid mass (tumor) that doesn't perform whatever function the original cells were supposed to perform. 

Eventually, the tumor gets big enough that it interferes with the normal functioning of the cells around it. If it gets big enough and is in a bad enough area (say the inside of a vital organ like the liver or pancreas) it can stop the organ from doing it's job, and the patient dies. 

Usually though, cancer patients die from a metastasis, which is when cancer cells break off from the main tumor into the blood and move to a new part of the body, where they start reproducing all over again and form a new tumor. 

This is why catching cancer early (before it metastasizes) is the most important factor in defeating it. If there is only one tumor and it's in a place that can be accessed by a surgeon, it can be removed. If there are lots of tumors, odds are some of them won't be surgically removable. 

There is still hope if your cancer does metastasize though, because there are many new drugs that focus on teaching the immune system to kill the cancer. 

Remember the surface proteins up in the first paragraph? Some cancers express certain surface proteins that aren't usually found in the body, and several clinical trials have successfully trained the immune system to recognize those proteins and attack the cancerous cells. 

The FDA is expected to approve the first human gene therapy in the coming months, and it uses this technology to treat a rare and extremely deadly form of blood cancer that usually affects children. This treatment has many terrible side effects and in trials only worked on 82% of patients, but there are currently no better drugs for this disease. 

Interestingly, this trial used a now outdated technique that involves using a neutered HIV virus to deliver the treatment. Most trials conceived of today use CRISPR instead (which is superior in almost every way), but the time it takes to complete trials and file paperwork means that the treatment is just now reaching the point where it can be approved for everyone. This seemingly strange approval of an outdated treatment is the exception rather than the norm though, and really is just a reflection of how fast the industry is moving. 

Other clinical trials are currently working on doing a complete re-boot of the immune system to achieve a similar goal. They extract bone marrow (which contains the cells that eventually become white blood cells) and genetically modify the cells to attack a specific surface protein that the patient's cancer expresses. The cancer patient then undergoes an intense round of chemotherapy to kill off their old immune system, and the modified cells are put back into the body to multiply and create a bigger badder immune system. 

This has the potential to cure most cancers, but the technology is in the early stages and still needs many years 
of refinement. It's also very expensive and difficult to scale up because each patient needs an individually tailored treatment. 

TL:DR; Cancer is cells dividing uncontrollably, you die from it when all those extra cells stop your normal cells from doing their jobs. Metastasis is when cancer spreads to other parts of the body, and that's what usually kills you. 

New gene therapies have the potential to "cure" most cancers, but aren't quite ready for prime-time yet.