Introduction to Infectious Disease
What is an infectious disease?
A disease in general terms is any condition which interferes with the normal functioning of an organism. Disease are further classed as being infectious if they are caused by an invasion of a pathogen, and they can be transmitted via one host to another.
A host is an organism infected by a pathogen
An infection is occurring if a pathogen has entered a host, established itself and is replicating
Signs and symptoms usually result from damage to the tissues and organs of the host
It is important to note that a microorganism is not a pathogen unless it actually causes disease.
How have humans tried to manage infectious disease?
For centuries, humans have been actively trying to prevent, treat and manage different diseases. In identifying the specific cause of a disease, scientists have a series or steps which were developed by Robert Koch in 1884.
The potential pathogen must always be present when the disease occurs
The organism can be isolated from the host and grown in pure culture
When organisms from the pure culture are inoculated into a healthy, susceptible host and the disease develops, this is further evidence for a specific cause
The organism can then be re-isolated, grown in pure culture and compared with the organism first injected for confirmation
Koch's work essentially provided a basis for identifying the specific cause of infectious disease, however additional studies were performed which were necessary in gaining knowledge regarding pathogens.
What is a pathogen?
A pathogen is an infectious agent which causes disease. The four most common pathogen groups include:
What diseases do I need to know?
There are 10 infectious diseases in the ATBLY syllabus which you will be required to have sufficient knowledge about.
Ross River virus
Viral diseases of honeybees
Australian bat lyssavirus
Crown gall disease (plants)
Chytridiomycosis (amphibian chytrid fungus)
Phytophthora dieback (jarrah dieback)
What is transmission?
Transmission is the passing of an infectious disease from a host to another individual. Pathogens have the ability to be transmitted from host to host in a variety of ways.
It is important to distinguish between infectious and contagious. Diseases which are easily transmitted by close contact with an infected organism are referred to as contagious; a disease such as tetanus may still be infectious but not contagious.
What are zoonotic diseases?
Zoonotic diseases are infectious diseases which have the ability to be transmitted from one vertebrate group to another. For example, humans may be infected with avian or swine influenza viruses. Transmission of zoonotic diseases is usually through direct contact with infected animals, close contact or through indirect contact.
What are some key understanding points regarding pathogens?
It's important to note that most micro-organisms are not pathogens. A micro-organism is only pathogenic due to special characteristics of said organism, such as the ability to invade a cell type, produce a number of toxins and either avoid or cope with the infected host's immune system. Pathogens differ in their pathogenicity, or in other words, their ability to cause disease.
The intensity of the pathogen's effets are referring to its virulence, which is a measure of the severity of disease which it causes. Virulence factors assist a pathogen in invading a host, causing disease and evading host defences.
Individuals in a community may vary in their susceptibility to a pathogen based on certain characteristics, such as age, status of their immune system etc.
Most pathogens do not present symptoms in their host immediately following infection, due to a time lag known as the incubation period. This is the time between contraction of the fisease and the onset of symptoms.