If a germ is pathogenic, it can make you ill. That is the whole job of the word: not 'dirty', not 'strong', not 'from a hospital' — able to cause disease in a body. Doctors and scientists use it about bacteria, viruses, and sometimes fungi. They do not use it about people, and you will almost never hear it in ordinary chat; it belongs to labs, textbooks, and medical reports. The messy bit students miss is that most tiny living things on and in us are not pathogenic. Many bacteria in the gut help us. So the word is a sorting hat: this one causes disease; that one does not. The matching noun is pathogen — the germ itself. Pathogenic is the describing word you put in front of it.