Process of Infection Propagation

Agents of Infection
Infectious agents are the disease-causing organisms. The characteristics of an agent that enable it to spread include:


 * Pathogenicity - capacity to cause disease
 * Virulence - ability to invade and cause serious disease.
 * Toxicity - ability to harm by creating toxins
 * Invasiveness - ability to penetrate host tissue and survive for some time
 * Resistance to physical factors - extreme temperatures, radiation
 * Dose of infection - quantity of agent that penetrates the organism

Spread of Infection
The principle of infection spreading depends on the ability of an agent to propagate. This criteria that determine this are
 * Source of the agent
 * The Mode of transmission of the agent
 * Susceptibility of host to agent

Source of Agent
The source refers to where an agent lives. It is from the host that the agent is expelled to the external environment. It isn't necessary that the agent causes disease in the host.

The host can be a:


 * Human during the incubation period (Hep A and B), the illness period or convalescence (e.g. diarrhoea).
 * Animal in a similar manner to humans. (e.g. sheep, goats, hens, small rodents)
 * The external environment (e.g. in legionnaire's disease, Mycoses)

Mode of Transmission
This is how the agent gets from its source to the host. It is the journey of the agent in the external environment. It is influenced by the way the agent is expelled fröm the source, it's ability to survive in the external environment (e.g. survival ability of spores) and the site of entry ito host.

This transmission can be Direct where the Host comes into contact with the agent.
 * Contact - skin or mucosa
 * Droplet - e.g. influenza
 * Vertically - mother to foetus e.g. toxoplasma gondii*
 * Perinatally - e.g. Neisseria gonorrhoea

Transmission can also be Indirect


 * Contaminated objects e.g. towels
 * Inoculation - e.g. injections
 * By air - small droplets dry immediately and can emain in atmosphere for a long time
 * Fecal-orally - by ingesting contaminated food or water.
 * Contaminated soil
 * Vector-borne - biological vectors like anopheles mosquito for malaria parasite, mechanical vectors like cockroaches contacting faeces.

Susceptibility of Host
This depends on
 * Degree of Immunity
 * Age when infected
 * Nutritional status
 * Present diseases
 * Amount of agents infecting host
 * Personal habits e.g. smoking, alcohol consumption

Prevention
Epidemiology aims to eradicate or eliminate infection and it's transmission. This can be done by
 * Liquidating the source of infection
 * Destroying agent in external environment by disinfection
 * Increasing the resistance of the population (e.g. vaccination) or preventing spread after infection (chemoprophylaxis)