A parasite is an organism that lives on or within another organism (the host) and benefits by deriving nutrients at the host's expense. A pathogen, on the other hand, is an organism (such as a virus, bacterium, fungus, or parasite) that causes a disease in the host. While both parasites and pathogens can harm their hosts, parasites do not always cause disease, and some parasites may even have mutualistic relationships with their hosts (benefiting both the parasite and host).
