What is disease?
A disease is defined as a physical or mental disorder or malfunction with a characteristic
set of signs or symptoms.
Diseases may be caused by a single factor such as a pathogenic microorganism or be
multifactorial and have many causes some of which may depend on lifestyle.
Pathogenic microorganisms include bacteria, viruses and fungi.
Each pathogen has a specific method by which it causes disease. Some such as the
influenza virus damage host cells whereas others, such as Vibrio cholera produce toxins
which have a harmful effect on the body.
The ability of the pathogen to cause disease depends on
o Location - what tissue is colonised
o Infectivity - how easily a bacterium can enter the host cell
o Invasiveness - how easily a bacterium or its toxin spreads wit