AS Bio notes Raida 12D
Infectious diseases
10.1 Infectious diseases
Infectious diseases are diseases that are caused by organisms known as pathogens
They are sometimes called Communicable diseases as they are passed from infected to
uninfected people - a process known as Disease transmission
‘Disease’ is often defined as an illness or disorder of the body or mind that leads to poor
health
Each disease is associated with a set of signs and symptoms
Many infectious diseases (ex. common cold, measles and flu) affect us for only a short
period of time
In the case of HIV/AIDS, there is as yet no cure, and treatments must be taken for the
whole of the person’s life
Some infectious diseases can only spread from one person to another by direct contact,
because the pathogen cannot survive outside the human body
Other pathogens can survive in water, human food, faeces or animals (including
insects), and so are transmitted indirectly from person to person
Some people may spread a pathogen even though they do not have the disease
themselves
People like this who are symptomless/asymptomatic, as they lack any symptoms, are
called Disease carriers
It can be very difficult to trace them as the source of an infection
The way in which a pathogen passes from one host to another is called the
Transmission cycle
Control methods attempt to break transmission cycles by removing the conditions that
favour the spread of the pathogen
Control is only possible once the cause of the diseases and its method of transmission
are known and understood
Vaccination is a major control of many infectious diseases
It works by making us immune to specific pathogens so that they do not live and
reproduce within us and do not spread to others, so breaking the transmission cycle
Diseases that are always in populations are described as Endemic (i.e TB is an endemic
in the whole human population, Malaria is endemic in tropical and sub-tropical regions)
Disease eradication: the complete breakage of the transmission cycle of a pathogen so
that there are no more cases of the disease caused by the pathogen anywhere in the
world
Terms that are also used to describe the occurrence of disease:
, Incidence: the number of people who are diagnosed over a certain period of time,
usually a week, month or year
Prevalence: the number of people who have that disease at any one time
Epidemic: occurs when there is sudden increase in the number of people with a disease
Pandemic: occurs when there is an increase in the number of cases throughout a
continent or across the world
Mortality rate: number of deaths over a particular length of time (usually a year)
Cholera
> Transmission:
Caused by the bacterium Vibrio cholerae
It is water-borne, thus occurs where people do not have access to proper sanitation
(clean water supply) and/or contaminated food
Infected people may be symptomless carriers, passing out large numbers of bacteria in
their faeces
If these contaminate the water supply, or if infected people handle food or cooking
utensils without washing their hands, then bacteria are transmitted to uninfected people
To reach their site of action in the small intestine, the bacteria pass through the stomach
If the contents are sufficiently acidic (pH less than 4.5), the bacteria are unlikely to
survive
However, if the bacteria do survive, they reach the small intestine, multiply and secrete
the toxin Choleragen
This toxin disrupts the functions of the epithelium lining the intestine, so that salts and
water leave the blood
The loss of salts and water causes severe diarrhoea
The loss of fluid can be fatal if not treated within 24 hours
> Treating:
Almost all people with cholera who are treated make a quick recovery
A death from cholera is an avoidable death
The disease can be treated quite cheaply by a solution of salts and glucose
If people can drink, they are given oral rehydration therapy
If not, then the solution is given intravenously to rehydrate the body
Glucose is effective, because it is absorbed into the blood and this is linked to the uptake
of sodium and potassium ions
It is important to make sure that a patient’s fluid intake equals fluid losses in urine and
faeces, to maintain the osmotic balance of the blood and tissues fluids
> Prevention:
Large cities that have grown considerably in recent years, but as yet have no sewage
treatment or clean water, create perfect conditions for the spread of the disease
Increasing quantities of untreated faeces from a growing population favour cholera’s
survival
These countries often do not have the financial resources to provide clean water
supplies and proper drainage
The use of raw human sewage to irrigate vegetables is a common cause of the disease,
as are inadequate cooking or washing in contaminated water
Cholera is now almost unknown in the developed world
As a result of sewage treatment and the provision of clean pipe water, which is
chlorinated to kill bacteria
Infectious diseases
10.1 Infectious diseases
Infectious diseases are diseases that are caused by organisms known as pathogens
They are sometimes called Communicable diseases as they are passed from infected to
uninfected people - a process known as Disease transmission
‘Disease’ is often defined as an illness or disorder of the body or mind that leads to poor
health
Each disease is associated with a set of signs and symptoms
Many infectious diseases (ex. common cold, measles and flu) affect us for only a short
period of time
In the case of HIV/AIDS, there is as yet no cure, and treatments must be taken for the
whole of the person’s life
Some infectious diseases can only spread from one person to another by direct contact,
because the pathogen cannot survive outside the human body
Other pathogens can survive in water, human food, faeces or animals (including
insects), and so are transmitted indirectly from person to person
Some people may spread a pathogen even though they do not have the disease
themselves
People like this who are symptomless/asymptomatic, as they lack any symptoms, are
called Disease carriers
It can be very difficult to trace them as the source of an infection
The way in which a pathogen passes from one host to another is called the
Transmission cycle
Control methods attempt to break transmission cycles by removing the conditions that
favour the spread of the pathogen
Control is only possible once the cause of the diseases and its method of transmission
are known and understood
Vaccination is a major control of many infectious diseases
It works by making us immune to specific pathogens so that they do not live and
reproduce within us and do not spread to others, so breaking the transmission cycle
Diseases that are always in populations are described as Endemic (i.e TB is an endemic
in the whole human population, Malaria is endemic in tropical and sub-tropical regions)
Disease eradication: the complete breakage of the transmission cycle of a pathogen so
that there are no more cases of the disease caused by the pathogen anywhere in the
world
Terms that are also used to describe the occurrence of disease:
, Incidence: the number of people who are diagnosed over a certain period of time,
usually a week, month or year
Prevalence: the number of people who have that disease at any one time
Epidemic: occurs when there is sudden increase in the number of people with a disease
Pandemic: occurs when there is an increase in the number of cases throughout a
continent or across the world
Mortality rate: number of deaths over a particular length of time (usually a year)
Cholera
> Transmission:
Caused by the bacterium Vibrio cholerae
It is water-borne, thus occurs where people do not have access to proper sanitation
(clean water supply) and/or contaminated food
Infected people may be symptomless carriers, passing out large numbers of bacteria in
their faeces
If these contaminate the water supply, or if infected people handle food or cooking
utensils without washing their hands, then bacteria are transmitted to uninfected people
To reach their site of action in the small intestine, the bacteria pass through the stomach
If the contents are sufficiently acidic (pH less than 4.5), the bacteria are unlikely to
survive
However, if the bacteria do survive, they reach the small intestine, multiply and secrete
the toxin Choleragen
This toxin disrupts the functions of the epithelium lining the intestine, so that salts and
water leave the blood
The loss of salts and water causes severe diarrhoea
The loss of fluid can be fatal if not treated within 24 hours
> Treating:
Almost all people with cholera who are treated make a quick recovery
A death from cholera is an avoidable death
The disease can be treated quite cheaply by a solution of salts and glucose
If people can drink, they are given oral rehydration therapy
If not, then the solution is given intravenously to rehydrate the body
Glucose is effective, because it is absorbed into the blood and this is linked to the uptake
of sodium and potassium ions
It is important to make sure that a patient’s fluid intake equals fluid losses in urine and
faeces, to maintain the osmotic balance of the blood and tissues fluids
> Prevention:
Large cities that have grown considerably in recent years, but as yet have no sewage
treatment or clean water, create perfect conditions for the spread of the disease
Increasing quantities of untreated faeces from a growing population favour cholera’s
survival
These countries often do not have the financial resources to provide clean water
supplies and proper drainage
The use of raw human sewage to irrigate vegetables is a common cause of the disease,
as are inadequate cooking or washing in contaminated water
Cholera is now almost unknown in the developed world
As a result of sewage treatment and the provision of clean pipe water, which is
chlorinated to kill bacteria
