Infectious diseases AS level Biology. Topic 10 CIE CAMBRIDGE

1. Infectious diseases
AS Bio

2. Infectious diseases
• A disease is an illness or disorder of the body or mind that leads to
poor health
• Infectious diseases are diseases caused by pathogens.
• These are also called communicable diseases as they can be spread
from infected to uninfected people (known as transmission)
• Some are passed from humans to animals

3. How infectious diseases
• Through direct contact-from one person to another
• Through indirect contact-spread through food, water, faeces, animals
• People who are carriers may spread a pathogen even though they do
not have the disease or symptoms themselves.

4. Terms used to describe the occurrence of a
disease
• Incidence of a disease - is the number of people who are diagnosed over a
certain period of time, usually a week, month or year.
• Prevalence of a disease is the number of people who have that disease at any
one time.
• An epidemic occurs when there is a sudden increase in the number of people
with a disease.
• Endemic disease-is a disease that is always there in a population
• A pandemic occurs when there is an increase in the number of cases throughout
a continent or across the world.
• Mortality rate – is the number of deaths over a particular length of time (usually
a year)

5. Definitions
• Signs- visible expression of the disease which can be found by
examining the patient e.g rash or high temperature
• Symptoms – an indication of a disease which is not detectable by
examination and can only be reported by the patient e.g headache
• Carrier - a person who has been infected but has no signs or
symptoms. The person can pass the disease onto another person
• Causative agent- the organism that causes the disease

6. The names and types of pathogens that cause
four infectious diseases
Disease Causative agent (pathogen) Type of pathogen
Cholera Vibrio cholerae bacterium (prokaryote)
Malaria Four species of plasmodium
 Plasmodium falciparum
 Plasmodium malariae
 Plasmodium ovale
 Plasmodium vivax
protoctist (eukaryote)
HIV/AIDS Human immunodeficiency virus (HIV) virus
TB Mycobacterium tuberculosis
Mycobacterium bovis
bacterium (prokaryote_)

7. Cholera
Pathogen Vibrio cholerae
Methods of transmission food-borne, water-borne
Global distribution Asia, Africa, Latin America
Incubation period two hours to five days
Site of action of pathogen wall of small intestine
Clinical features severe diarrhoea ‘rice water’, loss of water and salts,
dehydration, weakness
Method of diagnosis dipstick test of rectal swabs,
identification of V. cholerae in faecal samples using
microscopy
Annual incidence worldwide in 2017 1.3 million to 4.0 million (WHO estimate)
Annual mortality worldwide in 2017 21 000 to 143 000 (who estimate)

8. Method of transmission of cholera
• Cholera occurs where people do not have access to proper sanitation
(clean water supply)
• Drinking contaminated water or washing utensils in it
• It is also spread through eating contaminated food.
• Infected people, or symptomless carriers, pass out large numbers of
bacteria in their faeces.
• If the bacteria contaminate the water supply, or if infected people handle
food or cooking utensils without washing their hands, then bacteria is
spread to uninfected people
• The disease can spread rapidly in areas with inadequate treatment of
sewage and drinking water.

9. How cholera bacteria causes diarrhoea and
dehydration
• The site of action of cholera bacteria is the small intestine.
• If the bacteria reaches the small intestine, they multiply and secrete a
toxin choleragen.
• Choleragen disrupts the functions of the epithelium lining the
intestines so that salts (ions) and water leave the blood.
• This causes severe diarrhoea and the loss of fluid results in
dehydration.
• Muscle cramps result from the rapid loss of salts such as sodium,
chloride and potassium

10. Global distribution of cholera
• Cholera is common in developing countries
• Common locations include parts of Africa, south Asia, and Latin America.
Reasons
• Increase in population
• No sewage treatment and provision of clean water
• Raw sewage used to irrigate vegetables inadequate cooking or washing in
contaminated water increases the spread of bacteria
• Natural disasters such as cyclones, earthquakes and flood which disrupts the sewage
systems and cholera bacteria easily contaminates water
• Cholera is less common in the developed world
-Due to proper sewage treatment
- provision of clean piped water which is chlorinated to kill bacteria

11. Treatment and control of cholera
• Cholera can be controlled by oral rehydration therapy or a solution of
salts and glucose are given intravenously.
• This replaces the fluid lost in faeces and urine
• The glucose is absorbed into the blood and takes ions (e.g. sodium
and potassium) with it.
• Antibiotics such as tetracyclines and chloramphenicol are effective at
destroying bacteria

12. Prevention and control of cholera
• High standards of public and personal hygiene
• Proper sewage treatment and sanitation
• Clean piped water which is chlorinated to kill bacteria
• Vaccination of travellers from and to cholera endemic areas to reduce
the spread
• Health education

13. Malaria
Pathogen Plasmodium falciparum, P. vivax, P. ovale, P. malariae
Main method of transmission insect vector: female Anopheles mosquito (about 30 different species)
Global distribution throughout the tropics and sub-tropics (endemic in 106 countries
Incubation period from a week to a year
Site of action of pathogen liver, red blood cells, brain
Clinical features fever, anaemia, nausea, headaches, muscle pain, shivering, sweating, enlarged spleen
Method of diagnosis dipstick test for malaria antigens in blood microscopical examination of blood
Annual incidence worldwide in 2017 219 million cases of malaria – in 90 countries (WHO estimate) 92% of cases are in
Africa
Annual mortality worldwide in 2017 435 000 deaths (WHO estimate) 93% of deaths are in Africa

14. Transmission of malaria
• The female Anopheles mosquito bites an uninfected person passing
the infective stages of Plasmodium to an uninfected person.
• Female Anopheles mosquito feeds on human blood to obtain protein
to develop eggs.
• Plasmodium can also pass across the placenta from mother to foetus.
• Malaria may also be transmitted during blood transfusion and when
unsterile needles are re-used

15. Prevention and control of malaria
• Anti-malarial drugs like quinine and chloroquine are used to treat
infected people
• Chloroquine inhibits protein synthesis and prevents parasite from
spreading.
• Some drugs inhibit sexual reproduction of Plasmodium inside the
mosquito
• Prophylactics (preventative) drugs are used in stopping an infection if a
person is bitten by an infected mosquito.
• Prophylactics are taken before, during and after visiting an area where
malaria is endemic.

16. Prevention and control of malaria
• There are three main ways to control malaria
i) Reduce number of mosquitoes (use insecticides)
ii) Avoid being bitten by a mosquito (use mosquito nets and repellents)
iii)Use drugs to prevent parasite infecting people
• insect vectors can be killed to break the transmission cycle
• oil can be spread on surfaces of water to make it impossible for larvae
and pupae to breathe
• drain stagnant water sources
• clear vegetation

17. Biological control of mosquitoes
• Stocking ponds and water bodies with fish which feed on mosquito
larvae
• Spraying a preparation containing bacterium Bacillus thuringiensis
which kills mosquito larvae

18. Drug resistance in Plasmodium
• Where anti-malarial drugs have been used widely, strains of drug
resistant Plasmodium develop.
• The drug is no longer effective against the pathogen.

19. HIV Pathogen human immunodeficiency virus (HIV)
Main method of transmission -in semen and vaginal fluids during sexual
intercourse
-infected blood or blood products contaminated
-hypodermic syringes mother to fetus across
placenta and at birth
-mother to infant in breast milk
Global distribution worldwide, especially in sub-Saharan Africa and
South East Asia
Incubation period initial incubation a few weeks, but up to ten years
or more before symptoms of AIDS may develop
Site of action of pathogen T-helper lymphocytes, macrophages, brain cells
Clinical features HIV infection – flu-like symptoms and then
symptomless
AIDS – opportunistic infections including
pneumonia, TB and cancers; weight loss, diarrhoea,
fever, sweating, dementia
Method of diagnosis testing blood, saliva or urine for the presence of
antibodies produced against HIV
Estimated total number of
people living with HIV
worldwide in 2017
36.9 million (approximately 67% of these in sub-
Saharan Africa) (UNAIDS estimate
Estimated number of new cases
of HIV infection worldwide in
2017
1.8 million (UNAIDS estimate)
Estimated number of deaths
from AIDS-related diseases
worldwide in 2017
940 000 (UNAIDS estimate)

20. Mechanism of action of HIV
• HIV is a retrovirus-its genetic material is RNA, not DNA.
• The virus binds to receptors present in the surfaces of the T-lymphocytes
• It enters the lymphocytes by fusing with the cell surface membrane and injecting its
viral RNA directly into the cell
• Once inside a host cell, the viral RNA is converted ‘back’ to DNA (hence ‘retro’) by
reverse transcriptase to be incorporated into human chromosomes.
• The virus infects and kills the T-helper lymphocytes of the immune system
• The number of the T-helper cells gradually decreases
• The body is unable to defend itself against infections
• A range of pathogens invade the body and cause a variety of opportunistic infections
• AIDS is a collection of these opportunistic infections

21. Treatment of HIV
• There is no cure for AIDS and no vaccine for HIV
• Drug therapy can slow onset of AIDS.
• The drugs prevent the replication of virus inside host cells
• For example Zidovudine binds to the viral enzyme reverse
transcriptase and blocks its action.
• This stops replication of the viral genetic material and increases the
lymphocytes.

22. Prevention of HIV/AIDS
• Education of people about the spread of infection and behavioural
change to stop the spread
• Use of condoms
• Contact tracing -If a person who is diagnosed as HIV+
is willing and
able to identify the people he or she has put at risk of infection then
these people will be offered an HIV test
• Screen blood donated blood for HIV
• Mother to child transmission prevented by treating HIV positive
pregnant women with drugs

24. TB
Pathogen Mycobacterium tuberculosis; Mycobacterium bovis
Methods of transmission airborne droplets (M. tuberculosis); via undercooked meat and
unpasteurised milk (M. bovis)
Global distribution worldwide
Incubation period a few weeks or up to several years
Site of action of the pathogen primary infection in the lungs, secondary infection in lymph nodes, bone
and gut
Clinical features racking cough, coughing blood, chest pain, shortness of breath, fever,
sweating, weight loss
Methods of diagnosis rapid molecular test detecting presence of DNA from M. tuberculosis
microscopical examination of sputum for bacteria chest X-ray long-term
culture of bacteria (up to 12 weeks)
Annual incidence worldwide in 2017 10 million (WHO estimate) Over 90% of cases among adults; more men
than women
Annual mortality worldwide in 2017 1.6 million, including about 300 000 deaths of people who were HIV+
(WHO estimate

25. Transmission of TB
• TB is air-borne
• TB spreads when infected people with the active form sneeze or cough and bacteria
are carried in air in tiny liquid droplets
• Uninfected people inhale the droplets and get infected
• TB spreads rapidly in overcrowded conditions with poor ventilation
• People who sleep close together in large numbers are particularly at risk.
• The disease primarily attacks the homeless and people who live in poor, substandard
housing
• Low immunity because of malnutrition or being HIV-positive also increases the risk
of TB infection
• TB caused by M. bovis occurs in cattle and is spread to humans in meat and milk

26. Reasons for increase in TB cases
• Some strains of TB are now resistant to drugs due to partial treatment
• Increase of the HIV/AIDS pandemic. TB is the first opportunistic
infection to strike HIV positive people
• Poor housing in inner cities and homelessness in the developed world
• Breakdown of TB control programmes
• Partial control of TB leading to increased drug resistance
• Poor nutrition or those with low immunity are affected in the
developing world

27. Treatment of TB
• Use of antibiotics to kill bacteria
• Several drugs are used to ensure that all bacteria are killed

28. Drug resistance
• Drug resistance in bacteria occurs due to random mutation in bacterial DNA
• Antibiotics kill those bacteria that are not resistant and leave the drug resistant strains behind
• If TB is not treated or the person stops treatment before the bacteria is completely eliminated,
the bacteria spread throughout the body increasing the likelihoods of mutations arising
• People who do not complete the treatment are highly likely of infecting others with drug
resistant forms
• Multiple-drug resistant forms of TB (MDR-TB) now exist.
• MDR-TB strains of TB are resistant to at least two main drugs used to treat TB –isoniazid and
rifampicin which are known as first line drugs,
• WHO promotes a scheme that ensures that people complete their treatment course of drugs
course
• DOTS (direct observation treatment, short course) involve health workers and family members
ensuring that patients complete their course.

29. Prevention and Control of TB
• Contact tracing and subsequent testing of contacts
• BCG vaccination to protect children from getting the disease
• Routine testing for TB in cattle and killing all infected cattle
• Milk pasteurization to kill bacteria in milk

30. Antibiotics
• An antibiotic is a drug that kills or stops growth of bacteria without
harming cells of infected organism
• Penicillin is an example of an antibiotic

31. How antibiotics work

32. How penicillin acts on bacteria
• Bacteria have cell walls made of peptidoglycans held together by cross links that from between
them.
• Penicillin prevents the synthesis of the cross-links between the peptidoglycan polymers in the
cell walls of growing bacteria
• It inhibits the enzymes that build these cross-links.
• Growing bacterial cells secrete enzymes called autolysins which make little holes in its cell walls.
• The holes allow the wall to stretch so that new peptidoglycan chains can link together.
• Penicillin prevents the peptidoglycan chains from linking up but the autolysins keep making
holes.
• The cell wall becomes weaker
• The weakened cell walls cannot withstand pressure potential from exerted on them by the cell
contents and cells burst.

33. How penicillin acts on bacteria

34. Why penicillin does not affect viral cells
• Viruses do not have cell walls which are the targets for antibiotics
instead they are surrounded by a protective protein coat
• Viruses do not have cells

35. Antibiotic resistance
• Antibiotic resistance is the ability of bacteria or fungi to grow in the presence
of an antibiotic that would normally slow their growth or kill them
• Antibiotic resistance arises by mutation and becomes widespread when
antibiotics are overused
• Bacteria with the mutant gene in their DNA have a selective advantage.
• They will not be killed by the antibiotic and they survive and reproduce.
• Bacteria without mutant genes will be killed.
• Over time, the whole population of bacteria becomes antibiotic-resistant
because the antibiotic-resistant bacteria are best suited to their environment.
• This is an example of natural selection

37. Consequences of antibiotic resistance
• Misuse of antibiotics, increases the selection pressure on bacteria to
evolve resistance to antibiotics
• Antibiotic-resistant infections increase the risk of death. They are
often associated with long stays in hospital, and serious complications
• Widespread use of antibiotics on bacteria results in plasmids carrying
resistance genes for several different antibiotics, giving multiple
drugresistance.
• For example, methicillin-resistant Staphylococcus aureus (MRSA) is a
multiple drug resistant strain

38. How to reduce antibiotic resistance
• Using antibiotics only when appropriate and necessary; not prescribing
them for viral infections
• Reducing selling antibiotics without a doctor’s prescription
• Avoiding the use of so-called wide-spectrum antibiotics and using instead an
antibiotic specific to the infection (known as narrow spectrum)
• Making sure that patients complete their antibiotic course
• Making sure that patients do not keep unused antibiotics for self-medication
in the future or give them to someone else
• Changing the type of antibiotics prescribed for certain diseases so that the
same antibiotic is not always prescribed for the same disease
• Avoiding using antibiotics in farming to prevent, rather than cure, infections