Infection occurs when a harmful organisms or agents invade the body. The outcome of this depends on many factors including the host’s immune system. This section explains key terms, and the factors that influence whether exposure leads to infection.
Infection: pathogenic organisms or agents invade and/or colonise the body. The microorganisms then multiply on or inside the host. This may produce tissue injury and progress to disease.
Pathogen: an organism or agent (bacteria, virus, fungus, parasite) which can colonise, multiply and may invade a host, causing harm.
Host: is an organism (e.g. animal or human) that provides an environment for pathogens or parasites to survive, grow or reproduce.
The host may appear unwell as a result of the infection. However they can also be healthy, with no clinical signs, or may have previously shown signs and recovered. This type of host is called a carrier, a hidden source of infection that can shed pathogens and spread disease.
For a pathogen to cause an infection after entering the host, it must first overcome the host’s immune system. The likelihood of the exposure causing infection is influenced by several factors, including age, general health, vaccination status, the presence of wounds and the strength of the immune response. These elements play a crucial role in determining how effectively the body can resist or fight off invading pathogens.
Disease: When normal bodily functions are impaired, leading to specific and recognisable clinical signs. Infection is a common cause of disease, however there are other potential causes such as genetic, environmental, or a combination of multiple factors.
Zoonotic disease: a disease that can be passed from an animal to a human. Some diseases seen in veterinary practice are zoonotic. Being aware of these diseases and how to prevent their spread is vital to protect staff and owners/clients. Examples of zoonotic diseases include leptospirosis, ringworm, and giardia.
Do all micro-organisms cause infections?
Some bacteria and fungi live on humans and animals and do not cause disease, and in some cases can even be beneficial to their health.
Commensal: a microorganism which lives on the animal and gains a benefit (e.g. food) but does not cause harm, or benefit to the host. e.g. staphlococcus species on the skin.
Mutualism: beneficial association between the microorganism and the host, where both receive a benefit. e.g. gut flora
Some micro-organisms usually behave as commensals, but become pathogens if given the opportunity. For instance, normal flora found on the surface of the skin may cause an infection inside a wound (opportunistic infection). The opportunity may be a change in location on the body, or it could be other patient factors such as a weakened immune system, poor nutrition, stress etc. Commensals from one animal can also cause an infection in another individual given the opportunity. Infection control protects animals from pathogenic species but also opportunistic infections.