Infectious Diseases

An infectious disease is an illness caused by a pathogenic microorganism, such as a bacterium, virus, fungus, or protoctist, that can be spread, directly or indirectly, from one person or organism to another. The organism that causes the disease is known as a pathogen.

### Characteristics of Pathogens

Pathogens are diverse, but they all exploit a host to replicate. The main types are:

* Bacteria: These are prokaryotic organisms. They possess a cell wall made of peptidoglycan, a cell membrane, cytoplasm with 70S ribosomes, and a circular loop of DNA. Not all bacteria are pathogenic; many are harmless or even beneficial. Pathogenic bacteria cause disease by destroying tissues or releasing toxins. Examples include *Vibrio cholerae* (cholera) and *Mycobacterium tuberculosis* (TB).

* Viruses: Viruses are non-cellular and are considered obligate intracellular parasites, meaning they can only replicate inside living host cells. A virus consists of genetic material (either DNA or RNA) enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope. They invade host cells and take over the cell's metabolic machinery to produce more virus particles. An example is the Human Immunodeficiency Virus (HIV).

* Protoctista (or Protists): These are single-celled eukaryotic organisms. Some are pathogenic parasites that live on or in other organisms. A key example is *Plasmodium*, the protoctist responsible for causing malaria.

* Fungi: These are eukaryotic organisms, which can be unicellular (like yeasts) or multicellular. They cause disease by digesting and destroying cells, or by producing toxins. An example is *Tinea pedis*, which causes athlete's foot.

### Modes of Transmission

Pathogens are transmitted from an infected individual to an uninfected one through various routes:

* Direct Contact: Physical contact, including touching or sexual intercourse (e.g., HIV).

* Indirect Contact: Via contaminated surfaces or objects, known as fomites.

* Droplet Infection: Inhaling droplets of mucus or saliva expelled by an infected person coughing or sneezing (e.g., influenza).

* Waterborne: Consuming water contaminated with the pathogen (e.g., cholera).

* Vector: Transmission by another organism, such as an insect (e.g., the female Anopheles mosquito for malaria).

### Case Study 1: Cholera

* Pathogen: *Vibrio cholerae*, a bacterium.

* Transmission: Waterborne. The disease spreads through the ingestion of food or water contaminated with faeces from an infected person.

* Mechanism of Disease: Once ingested, *Vibrio cholerae* colonises the small intestine. It does not invade the intestinal wall but secretes a potent exotoxin called cholera toxin. This toxin binds to receptors on the surface of the intestinal epithelial cells. It causes a cascade of events that leads to the opening of chloride ion (Cl⁻) channels. A high concentration of Cl⁻ ions is secreted into the lumen of the small intestine. This significantly lowers the water potential of the intestinal contents compared to the blood and epithelial cells. Consequently, large volumes of water move by osmosis from the blood and tissues into the intestine, resulting in severe watery diarrhoea, extreme dehydration, and loss of essential electrolytes.

### Case Study 2: HIV/AIDS

* Pathogen: Human Immunodeficiency Virus (HIV), a retrovirus.

* Transmission: Through the direct exchange of infected bodily fluids, such as blood, semen, vaginal fluids, and breast milk. Common routes include unprotected sexual intercourse and sharing contaminated needles.

* Mechanism of Disease: HIV primarily targets and infects T helper cells (or CD4+ lymphocytes), which are critical components of the adaptive immune system. The virus has glycoproteins on its surface that bind to CD4 receptors on T helper cells. Once inside, the virus uses the enzyme reverse transcriptase to convert its viral RNA into DNA. This viral DNA is then integrated into the host cell's genome. The host cell then transcribes and translates the viral DNA, producing new virus particles that bud off to infect other cells, destroying the host cell in the process. Over time, the progressive destruction of T helper cells severely weakens the immune system. When the T helper cell count drops below a critical level, the individual is unable to mount an effective immune response against other opportunistic infections. This stage of the disease is known as Acquired Immunodeficiency Syndrome (AIDS).

### The Role of Antibiotics

Antibiotics are drugs used to treat bacterial infections. Their effectiveness relies on the principle of selective toxicity—they target structures or metabolic pathways in prokaryotic cells that are absent in eukaryotic (host) cells.

* Mechanisms of Action:

Antibiotics are ineffective against viruses because viruses lack their own cell walls, ribosomes, or metabolic pathways. They use the host cell's machinery for replication, so any drug that would inhibit viral replication would also harm the host's own cells.