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Chain of Infection (pdf) So we now know why it is important to understand the pathology of an infectious agent. But how do they spread and cause disease? In order for an infectious agent to successfully spread from one host to another, several conditions must be met. This is referred to as the chain of infection. If this ‘chain’ is broken at any stage, the infection cannot spread and becomes contained. We will use the basic human flu (influenza) as an example in this interactive below to explain the chain of infection. Infectious agent First there must be something that causes the disease. The flu is caused by an influenza virus. Reservoir A reservoir is the place the organism is found and sustained. This can be an animal or it can be something in the environment such as water, food and soil. Humans, birds and pigs serve as reservoirs for the influenza virus. Portal of exit
The infectious agent must then be able to leave the reservoir. Infected humans shed the virus through respiratory mucus, particularly through sneezing and coughing. Mode of transmission Following its exit from the reservoir, the infectious agent must be able to transmit to the host and survive the journey. Influenza virus can survive in bodily fluids for a limited time, and typically is transmitted via contact, and especially inhalation. After sneezing or coughing, infected respiratory fluids can either be directly inhaled by a nearly human (inhalation) or land somewhere which then comes into contact with another person’s eyes, nose or mouth e.g. tissue (contact). Modes of transmission Infectious agents can be spread in five main ways:
Contact (direct and indirect)
Direct skin to skin contact with contaminated bodily fluids can lead to transmission and subsequent infection. Contact can also be indirect, when infectious particles are able to survive on a nonliving object (fomite) for a period of time, such as a door handle or a used tissue.
Inoculation (bloodborne)
A type of contact transmission, this involves direct or indirect blood to blood contact. Often this occurs through sharing needles, or through cuts and other skin openings.
Ingestion
Also a type of contact transmission, the consumption of contaminated food and drink can lead to infection. If the infectious agent survives the digestion process, it can enter the body via the mucous membranes lining the gut. The infectious agent then replicates and exits via faeces. Inhalation (droplet and airborne) Infectious agents can cross the mucous membranes lining the respiratory system, and are shed from the host through respiratory mucous (a type of bodily fluid) expelled via sneezing, coughing, talking and even simple breathing. Larger infectious agents are transmitted via large, heavy drops of mucous (droplets) and can travel ~ 3 feet (91cm) from the infected person. Smaller infectious agents (<5 microns in size) can attach to dust particles and become airborne, travelling extreme distances via air currents.
Trans-placental
Trans-placental infections are those that are transmitted from the mother to her embryo or foetus via the placenta. The mother acts as the reservoir and the embryo is the susceptible host. Portal of entry Infectious agents gain entry into the host either through openings in the skin (e.g. cuts), or via mucous membranes lining the wall of the respiratory, gastrointestinal and genitourinary tracts. Infectious agents access the respiratory tract via the eyes, nose and mouth. Susceptible host In order to cause disease in the new host, this host must be susceptible to the disease. For example, the host can be naturally immune to the infectious agent, or be rendered immune via vaccination.
