SCIENCE: Disease and the Immune System

What is a disease?

A condition where normal bodily functions fail to function properly

What are diseases usually associated with?

Signs or symptoms

Categories of disease

- Infectious (e.g. as a result of pathogen)

- Environmental (e.g. as a result of pollution)

- Social (human induced)

- Genetic (caused by genetic abnormality)

- Deficiency (e.g. nutritional deficiency)

- Degenerative (as a result of ageing)

- Mental (neurological diseases)

Infectious vs. Non-infectious Diseases

Infectious diseases are caused by pathogens and can spread between individuals (e.g., flu, malaria). Non-infectious diseases result from genetics, lifestyle, or environment and cannot spread (e.g. cancer, diabetes).

Pathogen

A microorganism or agent that causes disease (viruses, protozoa, parasites, fungi, prokaryotes/bacteria, prions)

Viruses

Microscopic germs that have to infect a host to reproduce. Acellular (non-living)

Protozoa

Micro-organisms that have a membrane bound nucleus. Cellular (living)

Parasites

Organisms that live in, on or with another organism (host)

Fungi

Eukaryotic, cellular pathogens that decompose dead organisms or attack living things as their food source

Bacteria

Single-celled organisms that lack a nucleus; prokaryotes

Prions

A type of protein that can trigger normal proteins in the brain to fold abnormally. Acellular (non-living)

Cellular pathogens

Prokaryotes, protozoa, parasites, fungi

Acellular pathogens

Prions and viruses

Cellular vs. Non-cellular Pathogens

Cellular pathogens have cellular structures whereas non-cellular pathogens lack cells and require a host to replicate

Bacterial Reproduction

Bacteria reproduce asexually through binary fission, where a single cell divides into two identical daughter cells.

Viral Reproduction

Viruses infect a host cell, hijack its cellular machinery to replicate, and usually burst the cell to release new viruses.

Bacterial Shapes

Spheres (cocci), rods (bacilli), and spirals (spirilla).

Vector

An organism that transmits a pathogen from one host to another, such as mosquitos, fleas or rodents

Are all viruses, bacteria, fungi, protozoa and invertebrates pathogenic?

No

Epidemic vs. Pandemic

An epidemic is a sudden regional outbreak of disease whereas a pandemic is an epidemic that has spread across multiple countries or worldwide.

First Line of Defense

Non-specific physical and chemical barriers (like skin, tears, and stomach acid) that prevent pathogens from entering the body.

Second Line of Defense

Non-specific internal responses including inflammation, fever, and phagocytes

Phagocytes

White blood cells that engulf pathogens

Third Line of Defense

Specific immune response involving B cells (producing antibodies) and T cells (destroying infected cells) with memory cells for future protection.

How can diseases be spread?

1. Direct contact

2. Vector transmission

3. Vehicle transmission

Contact transmission

Airborne droplets, sexual contact or direct contact

Vehicle transmission

Food and waterborne, carried in blood

Vector transmission

Vector transmission requires an intermediate host. e.g. ex- mosquitos, fleas, rodents

Immunity

Resistance to a particular disease-causing pathogen

Active Immunity

Active immunity is acquired when your own immune system produces antibodies to a specific antigen (via infection or vaccination).

Passive Immunity

Passive immunity is when you receive antibodies from an outside source. This does not involve memory cells. e.g. breastmilk, placenta or antivenom shot

B cells

White blood cells that mature in the bone marrow and produce antibodies to target specific pathogens.

T cells

White blood cells that mature in the thymus and are involved in destroying infected or abnormal cells.

Memory cells

Cells that remember specific pathogens after a past infection or vaccination

Where do B and T cells mature?

In the bone marrow

Antibiotics

Medicines used to treat bacterial infections by killing bacteria or inhibiting their growth; they are ineffective against viruses.

Antibodies

Specialised proteins that aid in destroying infectious agents produced by your immune system

Antigen

A protein that, when introduced in the blood, triggers the production of an antibody

Methods to Stop Disease Spread

Personal hygiene, vaccination, quarantine, sanitation, and vector control.

Herd Immunity

When a large percentage of a population becomes immune, making the spread of disease from person to person unlikely.

Who does herd immunity specifically benefit?

The elderly, babies, pregnant women or people who are unable to be vaccinated

How do vaccines work?

Vaccines introduce a weakened or harmless part of a pathogen to stimulate the immune system, causing it to produce antibodies and memory cells without causing illness.

Are all antibiotics and disinfectants equally effective against all bacteria?

No