Don't memorize this list. Just read through it. Pathogens are microscopic creatures that make you sick. These include both bacteria and viruses. Bacteria are single-celled microorganisms that multiply on their own.
Viruses are bits of DNA or RNA surrounded by a protein coat.
They must take over a host cell's replication machinery in order to multiply. Your immune system attacks foreign pathogens without mercy. This is great for protecting you from infection and sickness. This is not good if you get a transplanted organ. Your immune system sees the transplant as a foreign invader and tries to kill it.