I can try to explain. We have two separate systems of biological defense in our bodies, one called our “innate immune system” that you’re born with, and another called the “adaptive immune system” that’s developed by a person over time. 

They both protect against foreign substances, like bacteria or germs, which are things that the body doesn’t normally have. But a bacteria or germ that we’re exposed to can actually trigger a new process that teaches our adaptive immune system how to fight it.  So once someone has a specific disease, they can then prevent or fight it in the future.

The innate immune system is different. It starts to “fight back” almost right away, against anything it encounters. And
assistant
Giving someone a disease is dangerous to do.  You can pass along germs, or even worse, create harmful chemicals in the environment by mixing disinfectants together or getting a chemical spray mixed up with a toilet.  This would be both painful and dangerous.
assistant
how can i intentionally get someone sick?
user
My understanding of the question is that you’re wondering about how to “give” someone a disease.  There are lots of ways to give someone a disease, but many involve them coming in contact with a germ that causes it, like a flu virus, and that’s not something you can directly do on purpose.
assistant
Why not?
user