Revise Infectious diseases for Cambridge 9700 (10.1)

An infectious disease is an illness caused by a specific type of organism, known as a pathogen, that can be transmitted from one host to another. These pathogens invade the body, reproduce, and cause damage or interfere with normal physiological functions. They are communicable, meaning they can spread.

It's important to differentiate between terms related to disease spread:

• Incidence: The number of new cases of a disease diagnosed over a certain period of time.
• Prevalence: The total number of people who have a disease at any given time.
• An endemic disease is consistently present in a specific geographical area or population at a relatively stable frequency, e.g., malaria in certain tropical regions.
• An epidemic refers to a sudden, rapid increase in the number of cases of a disease in a particular population over a short period, exceeding the expected level.
• A pandemic is an epidemic that has spread across multiple countries or continents, affecting a large number of people globally.
• Mortality: The death rate resulting from a disease.

Pathogens come in various forms, each with unique characteristics influencing how they cause disease and how they are treated. The Cambridge syllabus focuses on four main types:

1. Viruses: These are the smallest pathogens (typically 20-300 nm), non-cellular, consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid). They are obligate intracellular parasites, meaning they can only replicate inside living host cells, hijacking the cell's machinery to produce new virus particles. Examples include influenza virus and HIV.
2. Bacteria: These are prokaryotic, single-celled microorganisms (typically 0.5-5 µm) without a membrane-bound nucleus or organelles. They vary in shape (e.g., cocci, bacilli, spirilla) and have a cell wall made of peptidoglycan. They reproduce rapidly by binary fission. Some cause disease by producing toxins (exotoxins or endotoxins) or directly damaging host tissues. Examples include Salmonella and Mycobacterium tuberculosis.
3. Protoctista (Protozoa): These are eukaryotic, single-celled microorganisms, often motile. They are a diverse group, some of which are parasitic and can cause diseases like malaria (caused by Plasmodium species). They often have complex life cycles involving more than one host or a vector.
4. Fungi: These are eukaryotic organisms, which can be single-celled (yeasts) or multicellular (moulds). Pathogenic fungi often grow as a network of fine threads called hyphae, which form a mass known as a mycelium. They cause diseases by invading tissues, absorbing nutrients from host cells, or producing mycotoxins. Examples include Tinea pedis (athlete's foot) and Candida albicans (candidiasis).

For a disease to be infectious, its pathogen must be able to move from an infected host to a susceptible one. This process is called transmission, and it can be broadly categorised into direct and indirect methods.

Direct Transmission: Occurs when the pathogen passes directly from the source to the host without an intermediate step.

• Direct physical contact: Skin-to-skin contact (e.g., athlete's foot), sexual contact (e.g., HIV).
• Droplet infection: Pathogens expelled in tiny droplets of saliva or mucus (>5 µm) during coughing, sneezing, or talking, which are then inhaled by a nearby person (e.g., influenza, common cold).
• Mother to child (vertical transmission): Pathogens passing from a pregnant mother to her unborn child across the placenta, or during birth, or via breast milk.

Indirect Transmission: Occurs when the pathogen passes from the source to the host via an intermediate step or vehicle.

• Vectors: Living organisms (often insects) that transmit pathogens from one host to another. For example, female Anopheles mosquitoes transmitting Plasmodium (malaria).
• Contaminated food or water: Pathogens consumed through food or water that has been contaminated (e.g., cholera, salmonellosis).
• Airborne transmission: Pathogens suspended in air as small, dry particles or droplets (<5 µm) called aerosols, which can remain infectious for longer periods and over greater distances (e.g., tuberculosis, measles).
• Fomites: Inanimate objects (e.g., doorknobs, toys, clothing) that become contaminated with pathogens and can then transfer them to a new host upon contact.
• Contaminated soil: Pathogens present in soil entering the body through wounds (e.g., tetanus).

Malaria is caused by the protoctistan pathogen Plasmodium and is transmitted by a vector.

Transmission Cycle:

1. A female Anopheles mosquito, acting as a vector, feeds on human blood to obtain protein for egg development.
2. If the human is infected with malaria, the mosquito ingests Plasmodium gametes with the blood.
3. These gametes fuse in the mosquito’s gut and develop into infective stages (sporozoites).
4. The sporozoites migrate to the mosquito's salivary glands.
5. When the mosquito bites another human, it injects an anticoagulant from its salivary glands to prevent blood clotting. The sporozoites are injected into the human's bloodstream along with the saliva.
6. The parasites travel to the liver, where they multiply, and then infect red blood cells, causing them to burst, leading to the characteristic fever cycles of malaria.

Prevention and Control:

• Vector control: Reducing mosquito breeding sites by draining stagnant water or spraying oil on its surface. Using insecticides to kill adult mosquitoes.
• Reducing contact: Using insecticide-treated bed nets and insect repellents to prevent bites.
• Early diagnosis and treatment: Using rapid diagnostic tests (e.g., dipstick tests) for prompt identification and treating infected individuals with antimalarial drugs to break the transmission cycle.

Cholera is an acute diarrhoeal illness caused by the bacterium Vibrio cholerae.

Transmission and Pathogenesis:

1. The disease is transmitted via the faecal-oral route, typically through ingestion of food or water contaminated with the faeces of an infected person.
2. The bacteria pass through the stomach acid and reach the small intestine.
3. Here, they multiply and secrete a potent toxin called choleragen.
4. This toxin disrupts the function of the epithelium lining the small intestine, causing chloride ion channels to open.
5. Chloride ions move out of the cells into the intestinal lumen, lowering the water potential. Water follows by osmosis from the blood and tissues into the intestine, leading to severe, watery diarrhoea (often called 'rice-water' stool) and dehydration.

Prevention and Treatment:

• Treatment: The primary treatment is oral rehydration therapy (ORT), which involves drinking a solution of water, glucose, and salts to replace the lost fluids and electrolytes. In severe cases, intravenous fluids are required.
• Prevention: Ensuring access to safe, clean water (e.g., bottled, boiled, or chlorinated water), improving sanitation and hygiene practices like handwashing, and proper cooking of food.