Discuss the 5 elements that make up the chain of infection

Most of the infectious diseases follow a series of interactions that lead through a chain of infection to an infected host. The chain of infection is comprised of 6 elements.
1. The first link is the infectious agent, meaning the pathogen. The pathogen is the organism causing the disease, most of the time being a virus or a bacterium.
2.The second link is the reservoir. This term refers to the natural environment where the pathogen normally lives. Typical reservoirs could be any animal or a person or a plant.
3. The third link is the portal of exit, which consists the method used by the pathogen to leave its reservoir. For example, if its reservoir is a human being, the portal of exit could be its saliva or blood.
4. The fourth link is the mode of transmission. This refers to the way that the pathogen is transmitted to a host or another reservoir. This mode can be direct or indirect (through a vector).
5. The fifth link is the portal of entry. This refers to the method used by the pathogen to enter the host. It can be done in three different ways: penetration, inhalation or ingestion.
6. The final link is the susceptible host.

Neurotransmitters are messengers responsible for communication between neurons. they are released from synaptic vesicles and are received by receptors.  the communication is referred to as neurotransmission. the following statement is true about neurotransmitters: a. they're stored in synaptic vesicles in the axon.