Infectious diseases

Welcome to Topic 10: Infectious Diseases!

In this chapter, we dive into the fascinating, and sometimes scary, world of disease. We will learn about the tiny biological agents that cause illness—the pathogens—and how they move between people. Understanding how these diseases spread and how we fight back, both medically and socially, is crucial not only for your exam but for understanding global public health. Let’s get started!

10.1 Understanding Infectious Diseases

What is an Infectious Disease?

An infectious disease is an illness resulting from infection by a specific biological agent (a pathogen). The key feature, as the name suggests, is that they are transmissible—they can be passed from one host (like a human) to another.

Key Definitions

• Pathogen: A microorganism that causes disease.
• Infectious Disease: A disease caused by a pathogen and capable of being transmitted.
• Transmission: The mechanism by which the pathogen is passed from a source to a susceptible host.

The Main Types of Pathogens

Pathogens come in several forms. For AS Level Biology, you need to be familiar with three major types and their cellular structure (or lack thereof).

Quick Review Tip: Remember, only bacteria and protoctists are cellular organisms with their own metabolism. Viruses are non-cellular structures (Topic 1.2.7).

Key Takeaway (Section 1): Infectious diseases are caused by transmissible pathogens, which include bacteria, protoctists, and non-cellular viruses.

10.2 Four Major Infectious Diseases: Pathogens and Transmission

The syllabus requires specific knowledge about the pathogen and method of transmission for four critical diseases.

1. Cholera

Cholera is a severe diarrhoeal disease that can cause rapid dehydration and death if untreated.

• Pathogen Name: The bacterium Vibrio cholerae.
• Pathogen Type: Bacteria.
• Transmission:

  Vibrio cholerae is transmitted primarily via the fecal-oral route. This means the bacteria is shed in the faeces of an infected person and then ingested by a healthy person. This typically happens through:

  • Contaminated drinking water (the most common route).
  • Contaminated food (e.g., seafood or crops watered with contaminated water).

Analogy: Think of cholera transmission as drinking water that has been "recycled" improperly. Good sanitation breaks this cycle.

2. Tuberculosis (TB)

TB is a bacterial infection, usually affecting the lungs. It is highly contagious, especially in crowded, poorly ventilated conditions.

• Pathogen Name: The bacteria Mycobacterium tuberculosis or Mycobacterium bovis.
• Pathogen Type: Bacteria.
• Transmission:

  TB is transmitted via airborne droplets (droplet spread).

  • When an infected person coughs, sneezes, or talks, they release tiny droplets containing the bacteria into the air.
  • A healthy person then inhales these infected droplets.
  • M. bovis can also be transmitted from cattle to humans, usually via consumption of unpasteurised milk.

Did you know? TB is known as a disease of poverty because poor living conditions (crowding, lack of ventilation) drastically increase the rate of droplet transmission.

3. Malaria

Malaria is a life-threatening disease caused by parasites transmitted through mosquito bites.

• Pathogen Name: Protoctists of the genus Plasmodium (e.g., P. falciparum, P. vivax, P. ovale, P. malariae).
• Pathogen Type: Protoctists.
• Transmission:

  Malaria is transmitted by a vector, the female Anopheles mosquito.

  • An infected mosquito bites a human host, injecting Plasmodium parasites.
  • The parasites travel to the liver, reproduce, and then enter the red blood cells, causing symptoms like fever and chills.
  • A healthy mosquito bites an infected human and picks up the parasites, completing the cycle.

Crucial Note for AS Level: You need to know the pathogen and the transmission mechanism (mosquito vector), but details of the Plasmodium life cycle are NOT expected.

4. HIV/AIDS

HIV targets the immune system, leading to Acquired Immunodeficiency Syndrome (AIDS), which makes the body unable to fight off infections.

• Pathogen Name: Human Immunodeficiency Virus (HIV).
• Pathogen Type: Virus (specifically a retrovirus).
• Transmission:

  HIV is transmitted through the exchange of specific body fluids (blood, semen, vaginal secretions, breast milk).

  • Sexual activity (unprotected intercourse).
  • Sharing contaminated needles (e.g., for drug use).
  • Mother-to-child transmission (during birth or breastfeeding).
  • Blood transfusions (though rare in developed countries due to screening).

Key Takeaway (Section 2): Cholera is spread via contaminated water (fecal-oral); TB is spread via airborne droplets; Malaria uses a mosquito vector; and HIV is spread through specific body fluids.

10.3 Prevention and Control of Infectious Diseases

Controlling or eliminating a disease requires addressing it from several angles: biological, social, and economic.

1. Biological Factors (Targeting the Pathogen/Host)

These involve medical and environmental methods that directly interfere with the pathogen or boost the host's defense (Immunity, Topic 11).

• Drugs/Treatment: Using antibiotics (for bacteria like cholera/TB), anti-malarial drugs (for Plasmodium), or antiviral drugs (for HIV) to reduce the pathogen load in infected individuals.
• Vaccination: Protecting healthy populations (e.g., against some types of TB).
• Vector Control: Reducing the population of mosquitoes (for malaria) using insecticides, draining standing water, or using mosquito nets.
• Sterilisation/Hygiene: Treating water to kill Vibrio cholerae.

2. Social Factors (Targeting Human Behaviour)

These involve changing community behaviour and improving infrastructure.

• Health Education: Promoting safe sexual practices (condom use for HIV), hygiene (handwashing for cholera), and covering the mouth when coughing (for TB).
• Sanitation Improvement: Ensuring clean water supplies and proper sewage disposal systems (critical for preventing cholera).
• Quarantining/Isolation: Isolating infected individuals (especially important for reducing airborne spread of TB).
• Reducing Crowding: Improving living standards to decrease population density, limiting TB transmission.

3. Economic Factors (Resource Allocation)

Controlling disease requires significant financial resources.

• Infrastructure Cost: Funding safe water systems, sewage treatment plants, and reliable electricity (critical for refrigeration and sanitation).
• Healthcare Costs: Providing free or subsidised drugs (e.g., expensive HIV retroviral therapy or anti-malarials) and training healthcare workers.
• Development Aid: Wealthier nations funding research and prevention programmes in poorer nations where these diseases are often endemic (constantly present).
• Impact of Disease on Economy: Controlling disease boosts the economy by ensuring a healthy, productive workforce (a positive feedback loop).

Key Takeaway (Section 3): Effective disease control requires a multi-pronged strategy encompassing biological treatments (drugs, vaccines), social changes (education, hygiene), and economic investment (infrastructure, resource provision).

10.4 Antibiotics and Resistance

Antibiotics are miracle drugs of the 20th century, but their effectiveness is being seriously threatened. This section explains how they work and the risks associated with their misuse.

1. Penicillin: Mechanism of Action

Penicillin is a classic example of an antibiotic. Its action is highly specific to bacteria.

How Penicillin Kills Bacteria

1. Bacteria possess a strong, rigid outer layer called the cell wall, made of a molecule called peptidoglycan.
2. When bacteria divide, they must construct a new cell wall.
3. Penicillin works by inhibiting the enzyme responsible for cross-linking the peptidoglycan units.
4. The new wall is therefore weak and unstable.
5. Because the bacterial cell cytoplasm has a high water potential compared to its surroundings, water moves into the cell by osmosis, causing the cell to swell and burst (lyse).

Why Antibiotics Do Not Affect Viruses

Antibiotics are ineffective against viruses like HIV for two main reasons:

• Lack of Target Structure: Viruses are non-cellular and do not have a cell wall made of peptidoglycan. Penicillin has nothing to attack.
• Reliance on Host Machinery: Viruses replicate inside the host’s cells, using the host’s ribosomes and enzymes. Antibiotics cannot target these host processes without harming the human patient.

Memory Aid: Penicillin Pokes holes in Peptidoglycan, causing cells to Pop!

2. Consequences of Antibiotic Resistance

Antibiotic resistance occurs when bacteria develop mechanisms to withstand the effects of antibiotics. This is a crucial real-world application of Natural Selection (Topic 17).

The consequences of resistance are severe:

• Treatment Failure: Infections (like TB or pneumonia) become difficult or impossible to treat, leading to prolonged illness and death.
• Increased Spread: Resistant strains, such as MRSA (Methicillin-Resistant Staphylococcus aureus), spread rapidly in hospitals, leading to outbreaks.
• Higher Costs: Treating resistant infections requires more expensive, sometimes toxic, "last-resort" drugs and longer hospital stays.

Steps to Reduce the Impact of Antibiotic Resistance

We must act globally to slow the evolution and spread of resistant bacteria:

• Reduce Misuse in Humans: Only prescribe antibiotics when strictly necessary (e.g., not for viral infections like the common cold). Patients must complete the full course of medication to ensure all bacteria are killed.
• Reduce Use in Agriculture: Limit the use of antibiotics in farm animals, often used to promote growth, as this drives resistance.
• Develop New Drugs: Invest heavily in research to find new antibiotics and alternative treatments.
• Improve Hygiene: Better sanitation, especially in hospitals, prevents the transmission of resistant strains between patients.

Key Takeaway (Section 4): Penicillin disrupts the formation of the bacterial peptidoglycan cell wall. Viruses are unaffected because they lack this wall. Antibiotic resistance is a major public health crisis requiring responsible use in medicine and agriculture.