Single exposure to a common source
An example of this might be a feed or waterborne disease agent such as a chemical, a toxin, or microorganisms. A large number of animals are exposed at one time and the resulting disease episode is likely to be of sudden onset and of explosive nature. Such episodes are often referred to as outbreaks and give rise to outbreak investigations.
Multiple exposure to a common source
Similar to the above but groups of animals are exposed at different moments resulting in a number of disease episodes with a longer overall duration which therefore appears to be of less explosive nature.
A large number of animals may get exposed to a disease agent (particularly an infectious agent) when the agent is transmitted from one animal to a number of others. The resulting build-up of cases therefore depends on such factors as the rate of contact between animals, the length of the incubation period, etc.
Important terminologies in disease transmission
Cycle of transmission: the cycle of transmission includes the reservoir, how the agent leaves the reservoir, the mode of transfer to the new susceptible host, and how it enters the new host.
Vehicle and vector: the essential difference between a vehicle and a vector is that a vehicle is an inanimate object whereas a vector is a living animal.
Mechanical or biologic vector: a mechanical vector might be thought of as no more than a living vehicle, for the agent does not undergo any essential change during its association with the vector. The agent may be carried externally or internally. In biologic vectors the agent undergoes some type of change.
Transovarial and transstadial: vertical transmission is termed transovarial if the agent is transferred from the female to her eggs. Survival of the agent through stages of tick development from the nymph to the adult is termed transstadial transmission.
Serial interval: the time required to complete a cycle of transmission varies with each agent. The period from the appearance of signs in one case to the appearance of signs in a second case infected from the first, is known as the serial interval. This is an observable epidemiologic unit reflecting to some extent the life cycle of the parasite.
Speed of transmission: varies according to disease, route of transmission and level of herd immunity. The rate of spread is determined by such things as the number of sources, infectivity of the agent, and number of susceptible hosts.