system
Given the following post, respond as if you're commenting on Reddit:
human
Medicine: What actually kills someone with cancer?

You often hear that someone "died of cancer", be it breast cancer, prostate cancer, melanoma or something else but what is it that actually kills them? 

&#x200B;

Is it the immune system going havoc and killing all living cells it can find? Do you starve to death as the tumour grows and all nutrients you manage to ingest are seized by the cancerous cells? Infections, like pneumonia?
gpt
The easiest way to think of cancer is to think of the effect of weeds in a garden.

Weeds grow quickly and wherever they like, and they will choke out the plants you've intentionally put in your garden and either impede their function or kill them entirely.

In some cases, the weeds suck all the nutrients out of the soli and consume all the soil's water.  Plant dehydrates and starves and dies.

In other cases, especially with vines, the weed simply grows on top of the existing plants, preventing them from growing properly by simply occupying the spaces they would grow into - and in some cases, physically kills leaves and branches by squeezing them.

Metastatic cancer is when tumor cells from one area migrate to another area of the body - the bad ones are typically liver, brain, and lungs.  Metastatic breast cancer, for example, can cause malignant breast tissue to start growing in your lungs.  This means that normal lung tissue gets essentially choked out by the fast-reproducing cancer cells, and your lungs stop functioning as they should.

This is why chemotherapy has the side effects that it does.  Chemo targets fast-growing cells, like tumor cells and kills them.  But you have many other fast-growing cells in your body like hair folicles, stomach lining, etc.  Those tend to die too, so you lose your hair and have trouble eating.  It's also why cancer treatment follows the general course that it does.  You try to excise the tumor before it goes metastatic and migrates around the body.  Once it metastisizes, you can't surgically remove all those tiny tumor cells so you have to poison them, either chemically or by applying radiation in the areas that the tumors are detected.  If the poison doesn't effectively kill them all, you will eventually die.