system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, how come when you get an organ transplant, there's a chance your body will reject it and your immune system will kill it. But when you get infected by a new disease, it's difficult for your immune system to kill the disease.
gpt
When you get an organ transplant, your immune system can try to attack the organ, recognizing that it does not belong in your body. The same is true for infections. The difference is that an organ does not fight back, it does not try to spread and create more of itself. 

It’s like the difference between getting into a bar fight (an infection) and going into a nursing home and beating up old people (rejection)