system
Given the following post, respond as if you're commenting on Reddit:
human
What do tumors and cancers actually do thats so deadly?

What is different from like a cyst growing in the same place?
gpt
A cyst is like a little sac of fluid. If it gets very large it might push on the tissues near it, but that's unlikely. In any case it's just a single, well-contained thing. If necessary it can be drained or surgically removed. 

Some cancers are benign, and act similarly. They stay in one spot. At worst they cause some problems in the immediate area. 

Malignant cancer is very different, in two ways. 

First, a malignant cancer tends to grow without limit. This causes major problems in most of your organs. A tumor in your lung can displace healthy lung tissue, impairing breathing. A tumor in your colon can grow into the internal space and obstruct your bowels. A tumor in your brain can slowly squash the rest of your brain against the sides of your skull.

The second major difference is that malignant cancer does not just stay in one spot. Cancer cells split off and drift through the bloodstream, eventually settling into some other organ, where they begin to multiply and grow new cancer. This is called *metastasis*.

That's one of the things that causes danger even from cancers that may sound harmless. For example, melanoma is a kind of skin cancer. You may think that a growth on the skin isn't likely to interfere with anything life threatening. But if melanoma isn't treated early, its cells are likely to spread to places like the liver, lungs, or brain, causing new cancerous disease there. 

Metastasis is also one of the things that makes cancer so hard to treat. Even if you cut out all the cancer you can find in the original spot, there may be a few cells that have spread outside that region. You need to use more widespread treatments like chemotherapy or radiation to eliminate the stragglers, and if you don't get them all, there's always the chance for one to land in a favorable spot and start the problem over again.