The most famous one is the bacteria that causes the Bubonic Plague.
The cat with fleas is a host
A parasite typically requires two hosts: the definitive host and the intermediate host. The definitive host is where the parasite reaches maturity and reproduces, while the intermediate host is where the parasite undergoes development or larval stages. This two-host life cycle allows the parasite to complete its life cycle and spread effectively. Examples include the malaria parasite, which uses humans as the definitive host and mosquitoes as the intermediate host.
It is a parasite - host relationship. The fleas as parasites living off of the dog as a host.
Fleas jump to move from one host to another, they do not fly.
Cow: Intermediate host to the larval stages of beef tapeworm (Taenia saginata)
Fleas jump from host to host, so a flea can jump from one cat to another if bitten, but the bite itself will not cause the fleas.
a free-swimming larval stage in which a parasitic fluke passes from an intermediate host (typically a snail) to another intermediate host or to the final vertebrate host
Dogs/fleas is an example of a Host/parasite relationship.
http://tinyurl.com/gdhosting
vector
Intermediate host - snail definitive host - cattle/sheep
An intermediate host is an animal that is needed for a parasite to develop through one or more stages of life before being transferred to a definitive host in which the parasite will develop into sexual maturity. A reservoir host is a host that may be an intermediate or definitive host but provides a place where a parasite may "hang out" while enviromental conditions are less than optimal.