Final answer:
Horizontal transmission is how fleas, an ectoparasite, spread between animals in close proximity, typically through direct contact or a shared environment.
Step-by-step explanation:
The transmission of ectoparasites, such as fleas, occurs through horizontal transmission between animals in close proximity.
Horizontal transmission refers to the transfer of parasites or pathogens from one individual to another within the same generation, typically through direct contact or a shared environment.
In the case of fleas, which are a common ectoparasite, they can jump from one host to another when the hosts are close to each other, spreading infestations quickly through populations of animals.