A pathogen is a tiny living organism, such as a bacterium or virus, that makes people sick. Washing your hands frequently helps you avoid the pathogens that can make you sick.
Pathos is the Greek word for disease and -genes means "born of." So, a pathogen is something that causes disease, like a virus like the rhinovirus, which causes the common cold. At summer picnics, people are cautious about keeping certain foods like potato salad in coolers with ice — the eggs in such dishes spoil quickly out in the heat, introducing pathogens that can make people sick.