Biology
I
Disease
A disease is 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.
Infectious diseases: are diseases that are caused by organisms known as pathogens.
They are called communicable diseases as they are passed from infected to uninfected people.
Non-infectious diseases, as they are not caused by pathogens.
Inherited or genetic diseases, such as cystic fibrosis and sickle cell anaemia, deficiency diseases that
are caused by malnutrition and mental diseases.
Bacteria
Mycobacterium tuberculosis – tuberculosis (TB),
Vibrio cholorae - Cholera
Viruses
viruses – HIV/AIDS (human),)
Protoctista
protoctista – Plasmodium falciparum causes malaria,
Cholera
Cholera is an infectious disease that causes severe watery diarrhea, which can lead
to dehydration and even death if untreated.
Cholera Causes:
Vibrio cholerae, the bacterium that causes cholera, is usually found in food or water contaminated
by feces from a person with the infection. Common sources include:
Municipal water supplies
Ice made from municipal water
Foods and drinks sold by street vendors
Vegetables grown with water containing human wastes
Raw or undercooked fish and seafood caught in waters polluted with sewage
Cholera Symptoms
Symptoms of cholera can begin as soon as a few hours or as long as five days after infection.
The cholera bacterium produces a toxin that causes secretion of chloride ions into the small
intestine, causing osmotic movement of water into the gut, causing diarrhoea, dehydration and
loss of salts from blood
About one in 20 people infected have severe watery diarrhea accompanied by vomiting, which
can quickly lead to dehydration
Rapid heart rate
Loss of skin elasticity (the ability to return to original position quickly if pinched)
Dry mucous membranes, including the inside of the mouth, throat, nose, and eyelids
Low blood pressure
Treatment
Cholera is an easily treatable disease. The majority of people can be treated successfully
through prompt administration of oral rehydration solution (ORS). Adult patients may require
up to 6 L of ORS to treat moderate dehydration on the first day.
Individuals are given Oral cholera vaccines.
A course of antibiotics.
Prevention
sewage treatment and the provision of
clean piped water, which is chlorinated to kill bacteria.
The transmission cycle has been broken.
Travellers from areas free of cholera to those where cholera is endemic used to be advised to be
vaccinated, although the vaccine only provided short-term protection.
Malaria
Vector – organism which carries a disease from one person to another or from an animal to a
human e.g., female anopheles mosquito for plasmodium
Transmission
1) blood transfusion
2) when unsterilised needles are used
3) plasmodium can pass across placenta from mother to foetus
How infection of malaria occurs
1) female anopheles mosquito feeds on human blood to obtain the protein they need to develop
eggs
2) if an infected person is bitten, the mosquito takes up the pathogen’s gametes with the blood
meal
3) male and female gametes fuse in the mosquito’s gut and develop into infective stages that
move to the salivary glands
4) when the mosquito feeds again, an anticoagulant is injected from the salivary glands causing
the infective stages to pass out as well
5) the anticoagulant is injected to prevent blood from clotting
6) parasites enter the blood stream, then enter liver cells and finally infect red blood cells
Prevention and control
1) control of breeding of mosquitoes (e.g., drainage of stagnant water, aerial spraying of
insecticide, oil on water, stocking ponds)
2) reduction of contact between vector and humans (e.g., bed nets impregnated with
insecticide / insect repellents)
3) earlier identification of cases (introduction of dip stick tests ensuring diagnosis can be done
quickly)
4) use of new drugs to prevent transmission or using drugs in combination to reduce chances of
drug resistance arising
5) better awareness
6) better screening of blood before transfusion
Why malaria is very difficult to control
1) no vaccine as plasmodium is eukaryotic and antigens differ in the different life stages
2) drug resistance in Plasmodium
3) insecticide resistance in Plasmodium
4) cost of drugs
5) problems with funding research
6) lack of knowledge
7) infected people not identified
8) inaccessibility of some regions to healthcare
Why is it difficult to create a vaccine for malaria? 1) plasmodium is eukaryotic
2) antigens differ in different life stages
3) intracellular parasite
4) antigenic concealment
HIV/AIDS (acquired immune deficiency
syndrome
Transmission
1) semen and vaginal fluids during intercourse
2) infected blood (via donation, etc)
3) sharing needles
4) mother to foetus across placenta
5) mother to infant in breast milk
Symptoms
fever
headache
rash
sore throat
The infection progressively weakens the immune system. This can cause other signs and symptoms:
swollen lymph nodes
weight loss
fever
diarrhoea
Without treatment, people with HIV infection can also develop severe illnesses:
tuberculosis (TB)
meningitis
severe bacterial infections
cancers such as lymphomas and Kaposi's sarcoma
How HIV affects the body
1) genetic material of HIV is RNA
2) viral RNA is converted to DNA inside host cells to be incorporated into human chromosomes
3) infects and destroys T Helpers cells (cells that control the immune system’s response to
infection)
4) when the number of T Helpers cells are low, the body is unable to defend against infection
allowing pathogens to opportunistic infections
5) AIDS isn’t a disease – it’s a collection of opportunistic diseases associated with the
immunodeficiency caused by HIV infection
Treatment
1) drug therapy (e.g., zidovudine)
2) it binds to viral enzyme reverse transcriptase and blocks its action
3) this stops the replication of viral genetic material and blocks its action and leads to an
increase in the body’s lymphocytes
4) combination therapies are however difficult to follow
Prevention and Control
1) spread awareness
2) use condoms, femidoms and dental dams
3) don’t have many sexual partners
4) don’t share needles
5) contact tracing
6) blood collected by donors screened and heat treated
7) reduce mother to child transmission by using formula
Tuberculosis
Pathogen
Type of pathogen
Transmission
Site of action
Method of action
Mycobacterium
tuberculosis Mycobacterium bovis
Bacteria
M. tuberculosis: aerosol infection.
Infected person sneezes and the
uninfected person breathes in the
airborne droplets. M. bovis: from infected
cattle eg. unpasteurized milk
Lungs
Slow infection. The disease can stay
dormant and become active later when
the immune system is weakened eg. by
HIV
Racking cough, shortness of
breath, coughing blood
Diagnosis
Chest X-ray, microscopical examination of sputum
Treatment
Combination therapy of several antibiotics to
prevent resistance.
Prevention
BCG vaccine, contact tracing, quarantine, pasteurise
milk
Global Distribution
Endemic due to high rate of migration, HIV/AIDS
prevalence, overcrowded living spaces
Global Problem
-WHO declared TB a global emergency 1993
-1/3 world population are infected
-Major problem with affordable therapy in some
countries
-Issue of generic drug manufacture
-American attack on pharmaceutical factory in
Somalia removed the only source of available
medication
Global TB
-8 million new cases every year
-1.3 billion infected
-9 million have active disease
-2 million die annually
-Sub Saharan Africa 300/100,000
-Fatality rate - 23%
-Fatality rate (HIV+TB) - >50%