What makes you sick from a cold? Is it evil scientist squirrels implanting radioactive acorns in our hearts? Tiny alien parasites? Brain strain? Germs from a person with a cold can be shared with another person through sneezes.
There are types of germs called viruses that poison cells and command them to make copies of the germs. Then some of the viruses go to infect other cells. Your body then makes mucus (or snot) to wash away the viruses. When you sneeze these germs cath air and could easily enter another person.
So squirrels did not perform surgery. But tiny one-celled organisms do invade your body, hijack your cells and catch a ride to make another person sick.