Infectious Diseases

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Cambridge International AS & A Level Biology (9700)

Infectious Diseases

The big idea: Cholera, malaria, TB and HIV/AIDS spread by different routes, so each needs its own mix of biological, social and economic measures. Antibiotics such as penicillin kill bacteria, not viruses, and every use selects for resistance.

Summary — What This Chapter Covers

Infectious diseases are caused by pathogens and are transmissible.
Pathogens of cholera, malaria, TB and HIV/AIDS.
How each of the four diseases is transmitted.
Biological, social and economic factors in prevention and control.
How penicillin acts and why antibiotics do not affect viruses.
Consequences of antibiotic resistance and steps to reduce it.

1. Infectious diseases

Pathogens

Four diseases and their causes

Infectious diseases are caused by pathogens (disease-causing organisms or viruses) and are transmissible: they can be passed from one host to another.

Disease	Pathogen	Type of pathogen
Cholera	Vibrio cholerae	Bacterium
Malaria	Plasmodium falciparum, P. malariae, P. ovale, P. vivax	Protoctist
Tuberculosis (TB)	Mycobacterium tuberculosis, Mycobacterium bovis	Bacteria
HIV/AIDS	Human immunodeficiency virus (HIV)	Virus

Transmission

How each disease spreads

Disease	Transmission
Cholera	Faecal–oral: drinking water or eating food contaminated with faeces containing V. cholerae, where sewage is untreated or mixes with drinking water; also food handled by infected people and raw shellfish from contaminated water. The bacteria multiply in the small intestine and release a toxin causing severe watery diarrhoea.
Malaria	By a vector, the *female Anopheles* mosquito*. She takes up Plasmodium* with a blood meal from an infected person; the parasite develops in the mosquito and moves to its salivary glands; it is injected into the next person she bites. Also by blood transfusion, shared needles and from mother to fetus across the placenta.
TB	M. tuberculosis: airborne droplets released when an infected person coughs, sneezes or talks, inhaled by others; most likely with prolonged close contact in crowded, poorly ventilated conditions. M. bovis: mainly from unpasteurised milk or undercooked meat from infected cattle.
HIV/AIDS	In body fluids: unprotected sexual intercourse; sharing needles (e.g. injecting drug users); transfusion of unscreened blood or blood products; from mother to child across the placenta, during birth or in breast milk.

Prevention and control

Biological, social and economic factors

Disease	Biological	Social	Economic
Cholera	Oral rehydration therapy treats dehydration; existing vaccines give only short-term, partial protection; many strains; people without symptoms can still carry and spread the bacteria	Sewage treatment and safe, chlorinated water supplies; boiling water; hand washing and hygienic food handling; outbreaks spread fast in refugee camps and after natural disasters	Clean water and sewage systems are expensive to build; poverty limits access
Malaria	Plasmodium	Education about bed nets and removing breeding sites; war and displacement disrupt control programmes; people may not seek treatment	Cost of drugs, nets, spraying and vaccination; lost workdays and school days
TB
HIV/AIDS

2. Antibiotics

Penicillin

How it acts, and why it does not affect viruses

peptidoglycan cross-links
Penicillin inhibits the enzymes that form these cross-links
osmosis

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Consequences and how to reduce the impact

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Infectious Diseases