Which pair of organisms can be in a parasite/host relationship

Some examples of pairs of organisms in a parasite/host relationship are:

- Humans (host) and ticks (parasite)
- Dogs (host) and fleas (parasite)
- Cows (host) and liver flukes (parasite)
- Birds (host) and lice (parasite)
- Fish (host) and parasitic worms (parasite)