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                                      Topic 10: Infectious diseases

10.1 Infectious diseases 

Students should be able to:

1) state that infectious diseases are caused by pathogens and are transmissible

2) state the name and type of pathogen that causes each of the following diseases:

• cholera – caused by the bacterium Vibrio cholerae

• malaria – caused by the protoctists Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale and Plasmodium vivax

• tuberculosis (TB) – caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis

• HIV/AIDS – caused by the human immunodeficiency virus (HIV)

3) explain how cholera, malaria, TB and HIV are transmitted

4) discuss the biological, social and economic factors that need to be considered in the prevention and control of cholera, malaria, TB and HIV

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1— Infectious diseases: definition and transmissibility

An infectious disease is any illness caused by a pathogen, an organism that lives in or on another organism (the host) and causes harm. Pathogens include bacteria, viruses, fungi and protoctists (protists). Because they can be transmitted from one host to another, infectious diseases are transmissible diseases. Transmission can occur in many ways — directly (by contact, droplets or bodily fluids) or indirectly (via contaminated food, water, air, or through vectors such as insects). In every case, infection occurs when the pathogen enters the host, evades its defences, reproduces, and causes damage to cells or tissues either directly or through toxins or immune responses.

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2 — Names and types of pathogens that cause key diseases

  • Cholera is caused by the bacterium Vibrio cholerae, a motile, comma-shaped, Gram-negative bacterium that produces an enterotoxin leading to severe diarrhoea.

  • Malaria is caused by protoctist parasites of the genus Plasmodium — mainly P. falciparum, P. malariae, P. ovale, and P. vivax. These are eukaryotic parasites that infect red blood cells.

  • Tuberculosis (TB) is caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis, both of which have thick, waxy cell walls rich in mycolic acids that resist desiccation and many disinfectants.

  • HIV/AIDS is caused by the human immunodeficiency virus (HIV), a retrovirus that contains RNA and uses the enzyme reverse transcriptase to make DNA copies inside host cells, integrating into the host genome.

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3 — Transmission of cholera, malaria, TB and HIV

Cholera spreads by the faecal–oral route, mainly through drinking water or food contaminated with Vibrio cholerae from infected human faeces; outbreaks are common where sanitation and clean water supply are inadequate.
Malaria is transmitted by female Anopheles mosquitoes, which act as vectors, transferring Plasmodium sporozoites from the salivary glands of the mosquito into a human’s bloodstream during feeding.
Tuberculosis spreads by airborne droplet infection when infected individuals cough, sneeze, or speak, releasing bacteria that are inhaled by others; crowded, poorly ventilated living conditions increase transmission.
HIV is transmitted through body fluids, such as blood, semen, vaginal secretions and breast milk, entering the bloodstream or mucous membranes of an uninfected person; major routes include unprotected sexual contact, sharing of contaminated needles, transfusion of infected blood and mother-to-child transmission during birth or breastfeeding.

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4 — Biological, social and economic factors in prevention and control of cholera, malaria, TB and HIV

Effective prevention and control of infectious diseases depend on understanding both the biology of each pathogen and the social and economic context in which infection occurs.

  • Cholera: Biological control relies on provision of safe water supplies, proper sewage treatment and personal hygiene to interrupt faecal–oral transmission. Social and economic challenges include poor sanitation infrastructure, lack of public health education and the cost of water treatment. Rapid rehydration therapy (oral or intravenous) and antibiotics reduce mortality.

  • Malaria: Control measures target the mosquito vector and human infection. Insecticide-treated bed nets, indoor residual spraying, draining breeding sites, and prophylactic antimalarial drugs can reduce transmission. Obstacles include insecticide resistance in mosquitoes, drug resistance in Plasmodium, limited access to healthcare, poverty and political instability in endemic regions.

  • Tuberculosis: Biological control depends on BCG vaccination, early diagnosis, and prolonged antibiotic therapy (usually a multi-drug regimen). Poor adherence to treatment fosters resistant strains. Social factors such as overcrowding, malnutrition, HIV co-infection and inadequate public health systems increase prevalence.

  • HIV/AIDS: Control relies on education, safe sexual practices, screening of blood donations, needle-exchange programmes and antiretroviral therapy (ART), which suppress viral replication. Social stigma, inequality, misinformation, and the cost of lifelong therapy all hinder effective control. Long-term success requires international cooperation, funding, and public awareness.

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10.2 Antibiotics

Students should be able to:

1) outline how penicillin acts on bacteria and why antibiotics do not affect viruses

2) discuss the consequences of antibiotic resistance and the steps that can be taken to reduce its impact

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5 — How penicillin acts on bacteria and why antibiotics do not affect viruses

Penicillin, a β-lactam antibiotic discovered by Alexander Fleming, inhibits bacterial cell wall synthesis by binding to and inactivating the enzymes (penicillin-binding proteins) that form cross-links in peptidoglycan, the main structural polymer in bacterial cell walls. Without proper cross-linking, the cell wall weakens, osmotic pressure causes lysis, and the bacterium dies. Viruses, however, lack cell walls and do not possess their own metabolic machinery; they replicate using the host cell’s enzymes and ribosomes. Consequently, antibiotics like penicillin, which target bacterial structures or metabolic pathways, are ineffective against viruses — antiviral drugs must instead interfere with specific viral enzymes or replication steps.

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6 — Consequences of antibiotic resistance and ways to reduce its impact

Antibiotic resistance arises when genetic mutations or horizontal gene transfer confer the ability to survive exposure to an antibiotic; resistant cells then proliferate, rendering treatments less effective. The widespread and often inappropriate use of antibiotics in human medicine and agriculture accelerates this selection. Consequences include longer illness durations, increased mortality, higher healthcare costs, and the emergence of multidrug-resistant “superbugs” such as MRSA (methicillin-resistant Staphylococcus aureus) and drug-resistant Mycobacterium tuberculosis. Reducing the impact requires judicious antibiotic use (only when prescribed and completing full courses), infection control in hospitals, development of new antibiotics and alternative therapies, global surveillance of resistance, and public education on hygiene and responsible drug use. Restricting antibiotic use in livestock and improving sanitation further limit the spread of resistant strains.

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