Disease causing organisms (How diseases are spread: (Sexual contact,…
Disease causing organisms
A Host is said to have an infection when an infectious agent grows and multiplies within its body or on its surface
An infectious disease occurs when there is a change in the health of part or all of a hosts body, so that is does not function normally because of the presence of an infectious agen
What is an infectious disease?
Infectious agents include viruses, bacteria, fungi, prions, viroids and protist
Prions are microscopic protein particles lacking nucleaic acid responsible for some degenerative diseases of the nervous system
Viroids are similar to but smaller than viruses and consist solely of a strand of RNA
Protists are a diverse group of single cell eukaryotic organisms, which are often motile
Characteristics of an infectious disease
Number of parasitic organisms associated with the host
The virulence of the parasite
This is ability of a parasite to infect a host and cause disease
High virulent organisms almost always cause disease
Less virulent organisms can only cause disease in weakened hosts or if present in overwhelming numbers
The host defences and resistance to the parasite
The ability to cause disease is called pathogenicity
Three virulence characteristics of a parasite
Invasiveness: ability of the pathogen to enter a host and spread to different tissue
Degree of invasiveness is determined by: Being effectively transported to host, via water, air, soil, or vector.
Adhering to, colonising, or invading the host
Multiplying and growing in or on the host and completing its life - cycle
Evading host defence mechanisms
Infectivity: the ability of the pathogen to establish a focal point for the infection (liver, lungs)
Pathogenic potential: The ability of the pathogen to cause disease symptoms because it posses the ability to damage its host. A major part of pathogenic potential is the ability to produce toxins that damage the host.
Stages of an infectious disease
Incubation period: Time between infection and the first signs of the disease. The length of incubation period depends on: Virulence of the pathogen, Initial number of pathogens, The growth rate of the pathogen, The immunity of the host and the hosts state of health
The prodromal period: A short period of mild generalised symptoms, not all infectious diseases have this period
Illness: symptoms are most evident during this period., which is the most severe disease stage, The hosts immune response is not fully developed at this stage and the host is most damaged during the illness stage
The decline stage: The host's immune response controls the pathogenic infection and the host gradually returns to normal
Convalescence: The host recovers from the pathogenic infection and tissues and systems are repaired
Viruses: A virus is an infectious agent that has a genome containing either DNA or RNA and can alternate between two distinct states, intracellular and extracellular
General virus structure:
Viruses are Submicroscopice
They consist of nucleic acid surrounded by a protein coat, and occasionaly other components . These particles are metabolically inert on their own,
Function: To carry nucleic acid from one cell to another so that a new call can become infected.
performs metabollic functions necessary for virus replication - can lead to lysis of the cell
Alternatively the genetic material of the virus can integrate with the host cell. As the genetic material is replicated so is the DNA of the virus
Edward Jenner - Used cowpox virus to immunize against smallpox
Potato blight: infection of the potato tubers, spores develop on the leave of infected potato plants- Spores can travel for miles.
How diseases are spread: