Revision NotesAS

infectious disease — A disease caused by an organism such as a protoctist, bacterium or virus.

These diseases, also known as communicable diseases, can spread from an infected person to an uninfected person, or from animals to humans. They are distinct from non-infectious diseases, which are not caused by pathogens. Like a computer virus spreading from one computer to another, an infectious disease spreads from one host to another.

pathogen — An organism that causes disease.

Pathogens can be viruses, bacteria, protoctists, or fungi. They invade a host organism and disrupt its normal physiological functions, leading to illness. Understanding the specific pathogen is crucial for effective treatment and control. Think of a pathogen as a saboteur that infiltrates a system (the body) and causes it to malfunction.

disease transmission — The transfer of a pathogen from a person infected with that pathogen to an uninfected person; transmission may occur by direct contact, through the air or water or by animal vectors, such as insects.

Transmission can be direct (e.g., sexual contact, airborne droplets) or indirect (e.g., contaminated food/water, vectors). Breaking the transmission cycle is a key strategy in disease control and eradication efforts. Imagine a relay race where the baton (pathogen) is passed from one runner (host) to the next, either directly or via an intermediary.

disease carrier — Person infected with a pathogen who shows no symptoms, but can be the source of infection in other people (not carrier of an inherited disease).

Carriers are particularly problematic for disease control because their lack of symptoms makes them difficult to identify and isolate. They can unknowingly spread the pathogen, contributing to wider outbreaks. A disease carrier is like a hidden messenger, delivering a harmful package (pathogen) without showing any outward signs of carrying it.

disease vector — An organism which carries a pathogen from one person to another or from an animal to a human.

Vectors are typically arthropods like mosquitoes or ticks, which transmit pathogens during feeding. They are not necessarily infected themselves but act as intermediaries. Controlling vectors is a crucial strategy for preventing vector-borne diseases. A disease vector is like a delivery service that transports a package (pathogen) from one sender (infected host) to a recipient (uninfected host).

transmission cycle — The passage of a pathogen from one host to another is continually repeated as the pathogen infects new hosts.

This cycle describes the continuous process of a pathogen moving between hosts, ensuring its survival and spread. Control methods aim to interrupt this cycle at various points, such as by vaccinating hosts or eliminating vectors. Think of a continuous loop or a chain reaction where each infection leads to the potential for more infections, perpetuating the pathogen's existence.

disease eradication — The complete breakage of the transmission cycle of a pathogen so that there are no more cases of the disease caused by the pathogen anywhere in the world.

Eradication is the ultimate goal for some diseases, meaning the pathogen no longer exists in nature. This is a rare achievement, requiring highly effective control measures and global coordination. Eradication is like completely deleting a harmful file from every computer system globally, ensuring it can never reappear.

endemic disease — A disease that is always in a population.

An endemic disease is consistently present at a predictable level within a specific geographical area or population. Its continued presence is maintained by ongoing transmission within that population. Malaria in tropical regions is an example. Like a background noise that is always present in a particular environment, an endemic disease is a constant presence in a population.

HIV — Human immunodeficiency virus.

HIV is a retrovirus that primarily infects and destroys T-helper lymphocytes, weakening the immune system. It is the causative agent of AIDS. Its genetic material is RNA, which is reverse transcribed into DNA upon infection. HIV is like a saboteur that specifically targets and dismantles the command center (T-helper lymphocytes) of the body's defense system.

AIDS — Acquired immunodeficiency syndrome.

AIDS is a collection of opportunistic diseases and conditions that occur when the immune system is severely weakened by HIV infection. It is not a single disease but a syndrome characterized by a low count of T-helper lymphocytes, making the body vulnerable to various infections and cancers. AIDS is like a castle whose walls have been so severely damaged (by HIV) that it can no longer defend itself against various invaders (opportunistic infections).

opportunistic infection — An infection caused by pathogens that take advantage of a host with a weakened immune system, as may happen in someone with an HIV infection.

These infections are typically harmless in individuals with healthy immune systems but can become severe or life-threatening when the host's defenses are compromised. They are a hallmark of AIDS, as HIV destroys T-helper lymphocytes, leaving the body vulnerable. Imagine a city with strong defenses (immune system) that normally keeps petty criminals (opportunistic pathogens) in check. If the defenses collapse, these criminals can run rampant and cause serious damage.

antibiotic — A substance derived from a living organism that is capable of killing or inhibiting the growth of a microorganism.

Antibiotics are drugs specifically targeting bacteria by interfering with their unique cellular processes, such as cell wall synthesis or protein synthesis. They are ineffective against viruses because viruses lack these bacterial targets. An antibiotic is like a specialized weapon that targets specific vulnerabilities in bacterial cells, leaving human cells unharmed.

antibiotic resistance — The ability of bacteria or fungi to grow in the presence of an antibiotic that would normally slow their growth or kill them; antibiotic resistance arises by mutation and becomes widespread when antibiotics are overused.

Resistance develops through genetic mutations in bacteria, allowing them to survive exposure to antibiotics. These resistant bacteria then reproduce, leading to a population dominated by resistant strains. Overuse and misuse of antibiotics accelerate this process, posing a significant public health threat. Antibiotic resistance is like a lock-picking skill that some bacteria develop, allowing them to bypass the 'lock' (antibiotic) that was designed to stop them.