INFECTIOUS DISEASES
DISEASES
What is a disease?
A disease is a particular abnormal condition of the body or mind that
negatively affects the structure or function of all or part of an organism,
leading to poor health.
Each disease is characterized and associated with a set of signs and
symptoms
Types of diseases:
1. Infectious diseases:
Also known as communicable diseases. These are diseases which are
caused by pathogens and can be transferred from one person to another
or one organism to another.
Examples include malaria, measles and influenza
2. Non infectious diseases:
Also known as non-communicable diseases. These are long term
degenerative diseases that are not caused by pathogens and therefore
cannot be transferred from one organism to another.
Examples are cancer, diabetes and asthma.
Occurrence of diseases:
 Endemic disease: is a disease that is always present in the population
but limited to a particular region. The infections spread at a normal or
expected level and there are no extra infections, therefore the disease
spread and rate is predictable.
Examples of such diseases are TB and malaria.
 Epidemic disease: is a disease causing an unexpected and rapid
increase in the number of people with a disease within a population. It
affects many people at the same time.
Examples are cholera and measles.
 Pandemic disease: is a disease that spreads across a wide geographical
area affecting a large number of people throughout the world or
continents. The disease affects the global population.
Examples are COVID-19 and Influenza
Types of diseases and their pathogens
 A pathogen is an organism that causes a disease
Disease
Organism
Type
Name
Cholera
Bacterium (prokaryote)
Vibrio cholerae
Malaria
Protoctist (eukaryote)
Four species of
plasmodium:
 Plasmodium
falciparum
 Plasmodium
malariae
 Plasmodium ovale
 Plasmodium vivax
Tuberculosis (TB)
Bacterium (prokaryote)
Mycobacterium
tuberculosis and
mycobacterium bovis
HIV/AIDS
Virus
Human immunodeficiency
virus (HIV)
Transmission of diseases
1. Cholera:
 The disease is water-borne and food-borne. As a result the disease
occurs in areas lacking proper sanitation (lack clean water supply)
and uncontaminated food.
 It is transmitted when people; bath or wash in contaminated
water, drink contaminated water, or eat food exposed to
contaminated water as the bacteria lives in water.
 faeces egested by infected people contain a large number of the
bacteria causing cholera and can contaminate water supplies
leading to the transmission of the bacteria
 Also if infected people handle food and cooking utensils without
washing their hands, the bacteria could be transmitted to
uninfected people.
2. Malaria:
 Malaria is caused by one of four species of the protoctist
Plasmodium; P.vivax ,P.falciparum, P.ovale, P.malarie.
 Malaria is transmitted to humans by an insect vector, the female
mosquito Anopheles.
 Female Anopheles mosquitoes feed on human blood to obtain the
protein they need to develop their eggs. If the person they bite is
infected with Plasmodium, the mosquito will take up some of the
gametes pathogen with the blood meal.
The male and female gamete fuses in the mosquitos gut where
cell division occurs to form thousands maralial parasite. The
infective stages develop and move to the mosquitos salivary
glands.
When the infected mosquito feeds again the infective stages pass
from the mosquitos salivary glands into the humans blood,where
the parasites enter red blood cells and mulptiply.
 Malaria can also be transmitted during blood transfusion and
when unsterile needles are re-used.
 Plasmodium can also pass across the placenta from mother to
child.
3. Tuberculosis ( TB)
 The form of TB caused by mycobacterium tuberculosis is
transmitted through air droplets.
 When infected people with the active form of the disease cough
or sneeze, bacteria enter the air in tiny droplets of liquid and
when uninfected people then inhale these droplets TB is
transmitted.
 TB therefore spreads more quickly among people living in
overcrowded conditions
 The form of TB caused by Mycobacterium bovis occurs in cattle
and is spread to humans through contaminated meat and
unpasteurised milk
 Very few people in developed countries now acquire TB in this
way, although meat and milk can still be a source of infection in
some developing countries.
 This disease attacks people living in poor conditions, those with
low immunity due to malnutrition or HIV.
4. HIV
 The HIV virus is not transmitted by a vector (unlike in malaria) and
the virus is unable to survive outside of the human body
 The virus can only be transmitted by direct exchange of body fluids
 This means HIV can be transmitted in the following ways:
o sexual intercourse
o blood donation
o sharing of needles used by intravenous drug users
o from mother to child across the placenta
o mixing of blood between mother and child during birth
o from mother to child through breast milk
Prevention and control of diseases.
i. Cholera
 In fast growing cities in developed countries, there are inadequate
infrastructure to provide proper drainage and sewage treatment due to lack
financial resources and funds creating perfect conditions for the spread of
cholera.
 Following natural disasters and wars cholera outbreak can occur due to
damage of sanitation systems or provision of poor sanitations in temporary
housing.
 Use of raw sewage to irrigate vegetables and washing in contaminated
water are also reasons for the transmission of cholera
Therefore prevention of cholera can occur through:
o Providing adequate sewage treatment infrastructure
o The provision of clean, piped water that has been chlorinated to kill
bacteria (as this occurs in developed countries, cholera is very rare
among them)
o vaccination programmes in areas where cholera is endemic
Cholera can be controlled by:
ii.
o Ready access to treatments such as oral rehydration therapy (a solution
containing glucose, salts and water)
o Monitoring programmes by the World Health Organisation (WHO)
o Using antibiotics in severe cases (to reduce the risk of antibiotic
resistance)
Malaria
ANTIBIOTICS
What is an antibiotic?
 Is a drug that kills or inhibits the growth of bacteria, without harming the
cells of the infected organism. They are produced from organisms or are
made synthetically by various chemical processes.
How does penicillin affect bacteria?
 Bacterial cell walls are composed of peptidoglycans (long molecules
of peptides and sugars)which are held together by cross-links that
form between them
 When a new bacterial cell is growing, it secretes enzymes known as
autolysins that create small holes in the bacterial cell wall. These
holes allow the bacterial cell wall to stretch, so that new
peptidoglycan chains can form cross-links together.
 Penicillin stops these cross-links forming by inhibiting the enzymes
that catalyse their formation.
 However, the autolysins keep creating holes in the bacterial cell wall,
making the walls weaker and weaker.
 As bacteria live in watery environments and take up water by
osmosis, their weakened cell walls eventually burst as they can no
longer withstand the pressure exerted on them from within the cell.
 This means penicillin is only effective against bacteria that are still
growing as autolysins no longer create holes and no more cross-links
between peptidoglycan molecules are formed once the growth of a
bacterium is complete.
Reasons why antibiotics do not act on virus
 antibiotics do not act on viruses as viruses cannot be targeted in any
of the ways that an antibiotic targets a bacterial cell.
 This is because viruses do not have cells or cell walls and therefore
cannot be attacked by antibiotics.
 Also a virus replicates differently compared to a bacteria. It uses the
host cell’s mechanisms for transcription and translation, so not even
these processes can be targeted as antibiotics do not bind to the
proteins that host cells use in these processes.
Antibiotic Resistance
 Antibiotic resistance is the ability of bacteria to grow in the presence of
an antibiotic that would normally slow their growth or kill them by
developing defences against antibiotics.
Consequences of antibiotic resistance
 The antibiotics used to treat diseases become less effective, leading to
longer hospital stays, higher medical costs and increase in the risk of
death.
 There is a constant race to find new antibiotics as resistant strains are
continuously evolving.
Reducing the impacts of antibiotics resistance
The following steps can be taken in order to reduce antibiotic resistance:
 Tighter controls in countries in which antibiotics are sold without a
doctor’s prescription
 Avoiding the overuse of antibiotics, by prescribing them only when
appropriate and necessary, not using them for viral infections.
 Test the bacteria first to make sure that they prescribe the correct
antibiotic
 Avoiding the use of wide-spectrum antibiotics and instead using an
antibiotic specific to the infection.
 Antibiotics should not be used in non-serious infections that the immune
system will ‘clear up’.
 Patients must not keep unused antibiotics for self-medication of such
non serious infections in the future or give them to someone else.
 The patient should finish the entire prescribed course(even
If they feel better after a few days) so that all the bacteria are killed, and
none are left to mutate to become resistant strains.
 The type of antibiotics prescribed being changed so that the same
antibiotic is not always prescribed for the same infections and diseases
(this reduces the chance of a resistant strain developing)
 Avoiding using antibiotics in farming to prevent, rather than cure,
infections.