Year 11 Biology: Foundations and Mechanisms of Infectious Diseases

Characterization and Classification of Disease

• Definition of Disease: A disease is characterized by any change to an organism that negatively impacts on the functioning of the organism.
• Categories of Disease: Disease can be categorized into two primary groups:     * Infectious: Diseases that are transmitted from one organism to another.     * Non-infectious: Diseases that cannot be transmitted between organisms.

Non-infectious Diseases

• Cause Factors: These diseases are caused by a wide variety of factors rather than pathogens:     * Carcinogens: Cancer-causing chemicals.     * Radiation: Exposure to high-energy radiation.     * Genetics: Faulty or non-functional genes.     * Environmental Hypersensitivity: Reactions to agents such as pollen or dust mites.     * Lifestyle Choices: Patterns often related to diet and exercise.     * Degeneration: The degeneration of organs and tissues over time.

Infectious Diseases and Pathogens

• Transmission: Occurs in some way from organism to organism.
• Causative Agents: All infectious diseases are caused by pathogens.
• Mechanisms of Disease: Once inside a host's body, a pathogen causes disease by:     * Destroying cells.     * Releasing chemicals called toxins that disrupt normal cell function.
• Microbial Composition of the Human body:     * The human body contains approximately $10 \text{ trillion}$ human cells.     * The body also contains approximately $100 \text{ trillion}$ microbial cells (bacteria, fungi, and protozoa).
• Requirements for Pathogenic Success: To cause disease, a pathogen must be able to:     * Cross the protective barriers of the host.     * Multiply inside the host.     * Avoid the host's immune system attempts to destroy them.

The Six Main Types of Pathogens

1. Bacteria

• Characteristics: Bacteria are unicellular, prokaryotic organisms that reproduce via binary fission.
• Shapes:     * Rod-shaped ($\text{bacilli}$).     * Spherical ($\text{cocci}$).     * Spiral-shaped ($\text{spirilla}$).     * Curved ($\text{vibrio}$).
• Biological Nature: They are living organisms capable of movement, utilization of energy, and reproduction.
• Damage Mechanisms:     * Produced Toxins: Poisonous chemicals that inhibit protein synthesis, damage membranes (interfering with material transport), or interfere with nerve function.     * Invasion: Invading host organs and tissues.     * Spore Formation: Some bacteria produce spores that survive for long periods and are highly resistant to tough environmental conditions. Spores germinate and replicate once they enter a host.

Common Bacterial Diseases

2. Fungi

• Characteristics: Fungi are heterotrophs categorized into two groups: yeast and filamentous moulds.
• Growth: They feed on organic matter by secreting enzymes to break it down.
• Reproduction: They form spores which spread easily through the air; records show upward of $100,000 \text{ per cubic meter}$ in the air at one time.
• Classification by Site of Growth:     * On the surface of the host.     * On superficial layers (e.g., skin and nails).     * In subcutaneous tissues (inner layers of the skin).     * Inside the host (infecting inner tissues and organs).

Common Fungal Diseases

3. Protists

• Characteristics: A diverse group of unicellular eukaryotic organisms.
• Parasitic Nature: Pathogenic protists are heterotrophs that absorb nutrients from their hosts.
• Groups by Site of Infection:     * Intestinal.     * Uro-genital (urinary and reproductive systems).     * Blood and tissue.

Common Protist Diseases

4. Viruses

• Characteristics: Described as infective particles rather than living organisms. They are extremely small compared to prokaryotic bacteria.
• Structure: Consist of nucleic acid ($\text{DNA}$ or $\text{RNA}$) packed into a protein coat.
• Infection Process: Either the entire virus enters the host cell or it attaches to the surface and injects genetic material.
• Replication Cycle: The host cell replicates thousands of new virus particles, which usually causes the cell to break open and die, discharging the particles.

Common Viral Diseases

5. Parasitic Worms

• Characteristics: Usually live in the host's intestine and feed on food providing nutrients.
• Health Impact: Symptoms range from minor to a considerable weakening of the immune system.
• Adaptations:     * Lack of digestive systems (in some species).     * Highly developed reproductive systems (laying many eggs passed in faeces).     * Complex life cycles often involving multiple hosts.

6. Prions

• Characteristics: Unusual infective agents of the nervous system. They are misfolded or misshaped protein molecules.
• Molecular Makeup: Identical amino acid makeup to a normal host protein but with a crucially different $3\text{D}$ structure.
• Infection Mechanism: Prions convert normal proteins into abnormally shaped proteins. Use of a microscope reveals holes in the cortex of the brain (lesions).

Common Prion Diseases

Disease Transmission Mechanisms

• Droplet Infection: Small droplets from the mouth or respiratory passages during coughing or sneezing. One cough/sneeze may release millions of microorganisms.
• Direct Contact: Transmission through physical contact. If a healthy person touches an infected area and then their own nose, the virus is transferred. Includes sexually transmitted diseases like genital herpes.
• Faeces: Water contaminated by human or animal faecal matter contains pathogens. This is a significant cause of death in low-income countries where clean water is scarce.
• Contaminated Food: Microbes feed on the same nutrients as humans. Ingesting food with high microbial levels can lead to poisoning via toxins or live pathogens. Symptoms include fever, cramps, and diarrhea.
• Animals:     * Direct Spread: Pathogens growing in humans and animals (e.g., bats and monkeys harboring viruses; cats/dogs spreading bacteria via bites or licks).     * Vectors: Animals or insects that spread pathogens from one host to another without suffering from the disease themselves.
• Body Fluids: Pathogens thrive in the bloodstream and secretions. Transfer occurs via sexual intercourse, bleeding wounds, or sharing intravenous needles.

Detailed Examples of Parasite Life Cycles

Example 1: Malaria

• Pathogen: $Plasmodium$.
• Vector: Female $Anopheles$ mosquito.
• Mechanism: The parasite spends part of its life in humans and part in the mosquito. It multiplies inside human red blood cells, breaking them open and releasing more parasites.
• Impact: Causes hundreds of thousands of deaths annually; severe cases involve fever, coma, and convulsions.

Example 2: Tapeworms

• Main Host: Household cats.
• Intermediate Hosts: Rats, mice, and fleas.
• Adaptations:     * Suckers or muscular grooves on the head for intestinal attachment.     * Hermaphroditic: A single worm contains both ovaries and testes for reproduction.

Epidemics and Health Issues

• Definitions:     * Epidemic: A pathogen spreads more than usual in a short space of time, leading to widespread disease.     * Pandemic: A global spread of an epidemic.
• Causes of Epidemics:     * Increased virulence due to mutations/genetic changes.     * Transmission to a new group of humans with no previous exposure.     * Animal-to-human transmission (previously non-occurring).     * Low population immunity.
• Factors Influencing Spread:     * Mobility of populations: Cheaper international travel (e.g., the spread of AIDS).     * Intravenous Drug Use: Sharing needles allows fluid transfer.     * Poor Sanitation: Lack of clean water in low-income countries or refugee camps.     * Animal Reservoirs: Clearing vegetation or keeping exotic pets can expose humans to infected animals like monkeys and bats.

Evolutionary Adaptations and Virulence

• Virulence: The degree to which a pathogen causes disease. Pathogenicity is the ability to cause disease.
• Strategies:     * 'Hit and Run': Rapid infection and spread.     * 'Hit and Stay': Long-term survival inside the host while evading defenses (e.g., $HIV$ hiding in the lymphatic system).
• Genetic Structure: Mutations in viral $DNA$ alter protein coding, allowing pathogens to evade the immune system.
• Antigens: Surface proteins used to classify viral particles. There are thousands of potential combinations. The influenza virus frequently alters these to 'trick' the immune system, making previous immunity ineffective.
• Drug Resistance: Exposure to antibiotics allows resistant strains to survive and reproduce. 'Superbugs' are now resistant to almost every known antibiotic.
• Population Immunity: Protection arising when a large percentage of individuals are immune.     * Herd Immunity: Can be natural (infection) or artificial (vaccination). It protects both immune and non-immune individuals.

Disease Control Strategies

Controlling Vectors

• Sanitation: Eliminating breeding grounds (stagnant water) and practicing good food storage to control rats.
• Barriers and Traps: Insect repellents, mesh screens, bed netting, and manual or ultraviolet traps. Kerosene/oil on water prevents larvae growth.
• Insecticides: Chemicals like $DDT$, malathion, and pyrethrums.
• Biological Methods: Genetic manipulation (sterile males) and introducing predators (fish that eat larvae).

Killing Pathogens

• Antibiotics: Chemicals that kill or reduce bacterial growth. Discovered by Alexander Fleming ($1928$) via Penicillium mould.
• Antiseptics: Chemicals used on the outside of the body (e.g., alcohol, boric acid, hydrogen peroxide, iodine).
• Disinfectants: Used on surfaces (floors, toilets). They often oxidise or split open bacteria.

Other Measures

• Quarantine: Keeping infected individuals separated from the healthy population.
• Epidemiology: Analysis by scientists who study disease distribution and causes.

Advanced Pathogen Adaptations

• Entry Points: Humans are vulnerable through respiratory surfaces, wounds, the digestive system, and reproductive organs.
• Attachment: Production of specific proteins called $\text{adhesions}$ that bind to host cell molecules.
• Withstanding Harsh Environments: $Heliobacter\text{ pylori}$ (causes stomach ulcers) survives $pH\text{ }2$ by secreting an enzyme that converts urea to ammonia to neutralise acid.
• Biofilms: Over $99 \text{ percent}$ of bacteria exist in colonies with:     * Dormant states with reduced metabolism.     * Genetic info transfer for resistance.     * A slime-like matrix to prevent antibiotic penetration.
• Binding and Endocytosis: Viruses use surface molecules to bind to receptor molecules, allowing entry into the cell via endocytosis.
• Immune Evasion: $Mycobacterium\text{ tuberculosis}$ can survive inside white blood cells within a lesion for decades.
• Altering Host Behavior:     * Inducing diarrhea (e.g., Cholera) to spread via water.     * Coughing and sneezing: A single sneeze releases up to $20,000 \text{ droplets}$. If infected with rhinovirus, this carries thousands of particles.

Plant Pathogens

• Fungi: $Phytophthora\text{ cinnamomi}$ causes root rot, destroying a plant's ability to absorb water/minerals. Spores survive long-term in soil.
• Bacteria: Approximately $100$ species infect plants. $Pseudomonas\text{ syringae}$ reduces tomato yields and evades the plant immune system.
• Viruses and Viroids: Viroids are smaller than viruses and contain $\text{RNA}$ (e.g., potato spindle tuba viroid). Often transmitted by vectors.
• Nematodes: Small, wormlike creatures that damage root cells in food crops like potatoes and strawberries.