A-Level Biology: Communicable Diseases

Understanding communicable diseases is fundamental to biology, as it reveals how pathogens disrupt homeostasis and how organisms have evolved intricate defences. For you as an A-Level student, mastering this topic connects microbiology to immunology, ecology, and public health, providing a lens through which to view real-world challenges like pandemics and food security.

Pathogens and Mechanisms of Disease Transmission

Communicable diseases are illnesses caused by infectious agents that can be spread directly or indirectly from one host to another. The causative agents, or pathogens, fall into four main groups: bacteria, viruses, fungi, and protoctists. Each group has distinct characteristics and modes of causing disease.

Bacteria are prokaryotic, single-celled organisms. While many are harmless or beneficial, pathogenic species cause disease by damaging tissues or releasing toxins. For example, Mycobacterium tuberculosis destroys lung tissue, leading to tuberculosis. Viruses are non-living entities consisting of genetic material (DNA or RNA) enclosed in a protein coat. They are obligate intracellular parasites, meaning they must invade a host cell to replicate, disrupting normal cellular function. Influenza and HIV are viral diseases. Fungi are eukaryotic organisms that can be unicellular, like yeast, or multicellular, like moulds. They often infect plants and animals by penetrating tissues and absorbing nutrients, causing conditions such as ringworm or potato blight. Protoctists are a diverse group of eukaryotic, mostly single-celled organisms. Parasitic protoctists like Plasmodium, which causes malaria, invade host cells and multiply within them.

Transmission refers to how a pathogen moves from a source to a new host. Mechanisms can be direct, such as through physical contact (e.g., skin-to-skin for MRSA) or droplet spread (e.g., sneezing for the common cold). Indirect transmission involves an intermediate vehicle. This includes airborne transmission via dust or aerosol particles, as seen with tuberculosis; waterborne or foodborne transmission, typical for cholera; and vector-borne transmission, where an animal like a mosquito (malaria) or tick (Lyme disease) carries the pathogen. Understanding these routes is crucial for designing effective public health controls, such as sanitation, isolation, and vector eradication programs.

Plant Defence Mechanisms

Plants lack a mobile immune system but have evolved a sophisticated array of static and inducible defences to protect against pathogens. Their first line of defence consists of physical barriers. The waxy cuticle on leaves and stems repels water and blocks pathogen entry. Bark on trees and cellulose cell walls provide further structural protection. Some plants have adaptations like thorns or hairs that deter herbivores which might otherwise create wounds for pathogens to enter.

When physical barriers are breached, plants deploy chemical defences. Many produce compounds that are toxic or inhibitory to pathogens and herbivores. Tannins, found in species like oak trees, are bitter polyphenols that bind to proteins in the gut of herbivores, reducing digestion and deterring feeding. Alkaloids, such as caffeine in coffee plants, are nitrogenous compounds that can disrupt insect nervous systems. Plants may also secrete antimicrobial substances like phenols or antifungal proteins directly at infection sites.

A key inducible defence is the rapid deposition of callose. Callose is a polysaccharide, a $\beta -1,3$-glucan, that plants synthesize in response to physical damage or pathogen detection. It is deposited in the cell walls at sieve plates in phloem tubes to seal off infected areas, preventing the spread of pathogens through the vascular system. Callose can also strengthen cell walls around infection sites, forming papillae that block fungal hyphae. This response is a clear example of how plants can locally isolate threats despite their lack of a circulatory immune system.

Animal Non-Specific Defences

Animals, including humans, possess non-specific (innate) defences that provide immediate, general protection against a wide range of pathogens. These defences are always active and do not target specific pathogens. The first barriers are physical and chemical: the skin acts as an impermeable shield, while mucous membranes in the respiratory and digestive tracts trap pathogens in sticky mucus. A critical chemical defence is the action of lysozymes. These are enzymes found in secretions like tears, saliva, and mucus. Lysozymes hydrolyse the peptidoglycan bonds in the cell walls of many bacteria, causing them to lyse, or burst. This is a prime example of a non-specific, always-ready biochemical defence.

If pathogens breach these initial barriers, internal defences activate. The inflammatory response involves mast cells releasing histamine, causing localised vasodilation and increased capillary permeability. This leads to redness, heat, swelling, and the recruitment of phagocytic white blood cells, such as neutrophils and macrophages. These cells engulf and digest pathogens through phagocytosis. Fever, another non-specific response, can inhibit bacterial and viral replication. These innate mechanisms work rapidly to contain infections while the slower, specific adaptive immune response is mobilised.

Transmission and Control of HIV and Tuberculosis

Analysing specific diseases illustrates the interplay between pathogen biology, transmission routes, and control strategies. Human Immunodeficiency Virus (HIV) and Tuberculosis (TB) are globally significant communicable diseases with distinct profiles.

HIV is a retrovirus that attacks helper T cells, a crucial component of the adaptive immune system. Its primary transmission routes are through direct exchange of bodily fluids: unprotected sexual intercourse, sharing contaminated needles, and from mother to child during childbirth or breastfeeding. Control focuses on prevention and management. Barrier methods like condoms prevent sexual transmission, while needle-exchange programs reduce spread among intravenous drug users. Antiretroviral therapy (ART) is the primary control for infected individuals; it uses a combination of drugs to inhibit viral replication, preventing the progression to Acquired Immunodeficiency Syndrome (AIDS) and reducing viral load to make transmission less likely. There is currently no curative vaccine, though pre-exposure prophylaxis (PrEP) drugs can be taken by high-risk individuals to prevent infection.

Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. It is primarily an airborne disease; infectious droplets are released when an infected person coughs or sneezes. Control measures are multifaceted. Public health strategies include identifying and treating active cases to reduce transmission, and contact tracing. The BCG vaccine provides variable protection, especially against severe forms in children. Treatment involves a long course of specific antibiotics (e.g., rifampicin and isoniazid) to combat the slow-growing, often drug-resistant bacteria. Directly Observed Therapy (DOT), where a healthcare worker watches the patient take each dose, is used to ensure compliance and prevent the development of multi-drug resistant TB (MDR-TB), a major pitfall in control.

Common Pitfalls

1. Confusing pathogen types and their nature. A common error is classifying viruses as living organisms or bacteria as always harmful. Remember, viruses are non-living and require a host cell, while many bacteria are beneficial. Viruses cause disease by disrupting cellular processes from within, whereas bacteria often do so via toxin production or direct tissue invasion.

1. Overlooking the specificity of defences. Students sometimes conflate non-specific and specific defences. Lysozymes and inflammation are non-specific; they work against any pathogen. In contrast, antibodies produced by B lymphocytes are specific to a particular antigen. Clarify that plant defences are entirely innate (non-specific), lacking adaptive immunity.

1. Misunderstanding disease transmission routes. For instance, stating that HIV is airborne or that TB is vector-borne is incorrect. HIV requires direct fluid exchange, while TB is airborne. Always link the pathogen's biology to its transmission method; fragile viruses like HIV cannot survive long outside a host, influencing their transmission route.

1. Oversimplifying control measures. It's a mistake to think vaccination is the sole control for all diseases. For HIV, prevention through behaviour change and ART is key, as no effective vaccine exists. For TB, antibiotic treatment is central, but vaccine efficacy is limited. Effective control requires a tailored approach based on transmission dynamics and pathogen biology.

Summary

• Pathogens—including bacteria, viruses, fungi, and protoctists—cause communicable diseases through distinct mechanisms and are transmitted via direct contact, airborne routes, vectors, or contaminated substances.
• Plants defend themselves through physical barriers like the cuticle, chemical defences such as tannins and alkaloids, and inducible responses like callose deposition to seal off infected areas.
• Animal non-specific defences provide immediate protection, featuring physical barriers, chemical agents like lysozymes that break down bacterial cell walls, and internal responses like inflammation and phagocytosis.
• HIV is a virus transmitted via bodily fluids, controlled primarily through prevention, antiretroviral therapy, and PrEP, while Tuberculosis is a bacterial airborne disease controlled by antibiotics, vaccination, and public health surveillance.
• Accurate understanding requires distinguishing pathogen types, defence system specificity, and precise transmission routes to inform appropriate disease control strategies.