Biology (9700) | Infectious diseases

Welcome to Topic 10: Infectious Diseases!

Hey there! This chapter is incredibly important because it connects abstract biological principles directly to global health and everyday life. We're going to explore how diseases spread, who the culprits (pathogens) are, and how we fight back, especially focusing on the growing problem of antibiotic resistance.
Don't worry if this seems like a lot of information. We'll break down the major diseases one by one, focusing only on the specific facts required by the syllabus.

10.1 Infectious Diseases: The Basics

What is an Infectious Disease?

An infectious disease is an illness that results from a specific biological agent or its toxic products. There are two crucial definitions you need to know:

1. Pathogen: This is the biological agent that causes the disease. Pathogens are organisms (or structures, like viruses) that can cause harm to the host.
Examples of pathogens include bacteria, viruses, protoctists, and fungi.

2. Transmissible: This means the pathogen can be passed from one host (e.g., human) to another.
Think of a highly contagious cough—the disease is transmissible through airborne droplets.

Key Takeaway

Infectious diseases are caused by pathogens and are transmissible between hosts.

The Four Major Infectious Diseases (and their Pathogens)

The syllabus requires you to know the specific name and type of pathogen for four key diseases. It is essential to memorize these pairings accurately!

Memory Aid: Use the mnemonic C M T H (Cholera, Malaria, TB, HIV/AIDS) to remember the four diseases.

1. Cholera

Disease: Cholera
Pathogen Type: Bacterium
Pathogen Name: Vibrio cholerae

Did you know? Cholera primarily affects the small intestine, causing severe watery diarrhea and dehydration.

2. Malaria

Disease: Malaria
Pathogen Type: Protoctist (a type of single-celled eukaryote)
Pathogen Name: Plasmodium falciparum, P. malariae, P. ovale, and P. vivax (You must list at least one species, but knowing that the causative agents are species of Plasmodium is key).

3. Tuberculosis (TB)

Disease: Tuberculosis (TB)
Pathogen Type: Bacteria
Pathogen Name: Mycobacterium tuberculosis and Mycobacterium bovis

4. HIV/AIDS

Disease: HIV/AIDS
Pathogen Type: Virus
Pathogen Name: Human immunodeficiency virus (HIV)
Note: HIV is the virus; AIDS (Acquired Immunodeficiency Syndrome) is the collection of symptoms resulting from the immune system failure caused by the virus.

10.1.3 Modes of Transmission

Understanding how a disease moves from one person to the next is essential for controlling its spread. This is the "how" of the infection.

A. Cholera Transmission

Cholera is a classic example of a waterborne disease.

The transmission occurs when people ingest water or food contaminated with Vibrio cholerae bacteria, usually due to fecal contamination of drinking water sources (e.g., sewage leakage into rivers or wells).

B. Malaria Transmission

Malaria is a vector-borne disease.

The pathogen (Plasmodium protoctist) is transmitted via the bite of an infected female Anopheles mosquito (the vector). When the mosquito bites a human, it transfers Plasmodium sporozoites into the blood.

Analogy: The mosquito is like a dirty needle transferring the infection from one host to another.

C. Tuberculosis (TB) Transmission

TB is mainly an airborne disease (via droplet infection).

When an infected person coughs, sneezes, or talks, tiny droplets containing *Mycobacterium* bacteria are released into the air. If another person inhales these droplets, they can become infected. Prolonged close contact is usually required for transmission.

D. HIV Transmission

HIV is transmitted through the exchange of infected body fluids.

The main routes of transmission are:

1. Sexual contact: Through the transfer of semen or vaginal fluids.
2. Blood-to-blood contact: Sharing contaminated needles (e.g., drug users) or through transfusions with infected blood (less common today due to screening).
3. Mother-to-child: During pregnancy, birth, or breastfeeding.

Crucial Point to Remember: HIV is not transmitted by casual contact, such as hugging, shaking hands, sharing cutlery, or mosquito bites (unlike Malaria).

10.1.4 Prevention and Control: Biological, Social, and Economic Factors

Controlling infectious diseases requires more than just medicine; it needs a coordinated effort addressing three key areas:

1. Biological Factors (Dealing with the Pathogen/Vector)

These strategies target the biology of the disease agent or vector directly.

• Vaccination: Mass vaccination programmes (e.g., for TB in some countries) provide long-term immunity to the population.
• Drug Treatment: Using antibiotics (TB, Cholera) or antiretrovirals (HIV) to kill or inhibit the pathogen, reducing the infectious load in the community.
• Vector Control (Malaria): Biological control of mosquitoes, such as using larvae-eating fish or introducing sterile male mosquitoes, or using insecticides.
• Antiretroviral Drugs (HIV): These drugs reduce the viral load in an infected person, making transmission via sexual contact much less likely.

2. Social Factors (Education and Behaviour Change)

These address human behaviour and community organization.

• Education: Teaching communities about safe sexual practices (HIV), avoiding stagnant water (Malaria), and practising proper food/water hygiene (Cholera).
• Screening: Testing individuals for infection (e.g., blood screening for HIV before transfusions; Mantoux testing for TB).
• Isolation: Identifying and isolating highly infectious TB patients to prevent droplet spread.

3. Economic Factors (Infrastructure and Resources)

These involve the resources and wealth needed to implement successful control measures.

• Infrastructure (Cholera): Investing in clean water and sanitation systems (piped water, proper sewage disposal) to prevent fecal contamination.
• Cost of Drugs: Ensuring that essential medications (like expensive HIV antiretrovirals or malaria drugs) are affordable and accessible, especially in low-income regions.
• Housing Conditions (TB): Improving poor, overcrowded living conditions helps prevent the close-contact spread of airborne diseases like TB.

Key Takeaway

Effective disease control is holistic: it requires biological tools (drugs, vaccines), social education, and adequate economic resources (clean water, affordable medicine).

10.2 Antibiotics and Resistance

10.2.1 How Penicillin Works

Antibiotics are powerful drugs, but they only target certain types of pathogens, primarily bacteria.

Mechanism of Penicillin Action

Penicillin is a specific type of antibiotic that targets the bacterial cell wall.

1. Bacteria have a strong outer cell wall made of peptidoglycan (which eukaryotes, like us, do not have).
2. Penicillin inhibits the enzymes required by bacteria to synthesise (build) their peptidoglycan cell walls.
3. This is most effective when the bacteria are actively growing and dividing, as they are trying to build new walls.
4. Since the cell wall is weakened or incomplete, water enters the bacterium by osmosis, causing the cell to burst (lyse) and die. This is called a bactericidal effect.

Why Antibiotics Do Not Affect Viruses

This is a critical distinction!

1. Lack of Target Structure: Viruses are non-cellular structures (refer back to Topic 1.2.7). They do not have a cell wall made of peptidoglycan, nor do they have their own metabolic pathways or ribosomes.
2. Obligate Parasites: Viruses hijack the host cell's machinery (ribosomes, enzymes, ATP) for replication.
3. Since antibiotics like penicillin target bacterial structures (cell walls) or metabolic processes, they have no target in a virus or a virus-infected human cell.

10.2.2 The Problem of Antibiotic Resistance

Antibiotic resistance occurs when bacteria develop the ability to withstand the effects of antibiotics, making previously effective treatments useless. This is a massive global health crisis.

Consequences of Antibiotic Resistance

1. Treatment Failure: Infections become much harder or impossible to treat (e.g., drug-resistant TB or MRSA).
2. Increased Mortality: More people die from common infections.
3. Increased Healthcare Costs: Patients require longer hospital stays, more intensive care, and more expensive "last resort" drugs.
4. Reduced Effectiveness of Medical Procedures: Common operations, chemotherapy, and transplants rely heavily on effective antibiotics to prevent post-operative infection. Resistance makes these procedures riskier.

Analogy: Imagine your lock (antibiotic) used to keep the burglar (bacteria) out. But the burglar now has a new, modified key (resistance gene) that bypasses the lock entirely.

Steps to Reduce the Impact of Antibiotic Resistance

We need a multi-pronged approach to slow down the evolution and spread of resistant bacteria (a classic example of natural selection, Topic 17.2.4).

1. Minimise Use (Medical): Only prescribe antibiotics when absolutely necessary (e.g., confirm bacterial infection before prescribing).
2. Complete the Course: Patients must always complete the full prescribed course of antibiotics. Stopping early leaves the more resistant bacteria alive to multiply.
3. Reduce Use (Agriculture): Restrict the use of antibiotics in farmed animals to prevent resistance genes from entering the food chain.
4. Hygiene and Prevention: Implement strict hygiene practices (handwashing, sterile procedures) in hospitals and communities to prevent the transmission of bacteria in the first place.
5. Development of New Drugs: Invest heavily in research to find new antibiotics or alternative treatments (like phage therapy).