Germ Theory - Viruses and Bacteria Overview

By pjain      Published March 27, 2020, 8:57 p.m. in blog Health   

Part of Germ Theory Series

Microbes 101

Microbes are both good and bad. Microbes that cause disease are called pathogens.

There are many different types of Microbes!





Diseases and Microbes

Infectious disease Microbe Type
Cold Rhinovirus Virus
Chickenpox Varicella zoster Virus
German measles Rubella Virus
Whooping cough Bordatella pertussis Bacterium
Bubonic plague Yersinia pestis Bacterium
TB (Tuberculosis) Mycobacterium tuberculosis Bacterium
Malaria Plasmodium falciparum Protozoan
Ringworm Trichophyton rubrum Fungus
Athletes’ foot Trichophyton mentagrophytes Fungus

Key factors in Infection

Entry Portal

To cause an infection, microbes must enter our bodies. The site at which they enter is known as the portal of entry.

Microbes can enter the body through the four sites listed below: - Respiratory tract (mouth and nose) e.g. influenza virus which causes the flu - Gastrointestinal tract (mouth oral cavity) e.g. Vibrio cholerae which causes cholera - Urogenital tract e.g. Escherichia coli which causes cystitis - Breaks in the skin surface e.g. Clostridium tetani which causes tetanus

Target Site - access, attachment

To make us ill microbes have to: - Reach their target site in the body - Attach to the target site they are trying to infect so that they are not dislodged


  • Virus and Bacteria - rely on host for food and to provide the mechanism to propagate themselves
    • multiply rapidly
    • obtain their nutrients from the host

Immune system Response vs Microbes

  • Virus and Bacteria to be successful have to deal with the immune, specifically, avoid and survive attack by the host’s immune system

  • SRC

The Human Immune system

Skin primary line of defense - Cleanliness is Godliness!

  • The following image shows bacteria (blue and green) on human skin. If there are cuts in the skin, they can get into the body. So regular washing and disinfecting cuts is vital to prevent rapid progress of microbial infections.

Clots on skin prevents entry

If the skin is cut then the blood produces a clot which seals the wound and prevents microbes from entering.

Mouth, Nose, eyes Holes in your body - Mucous and Cilia protect

The respiratory system – the nose and passageways leading to the lungs – is lined with cells that produce sticky fluid called mucus that traps invading microbes and dust.

The mucous membranes act as a physical barrier preventing penetration by microbes.

Tiny hairs called cilia move in a wave-like motion and waft the microbes and dust particles up to the throat, where they are either coughed or sneezed out or swallowed into the digestive tract.

Digestive tract - Organ Linings and Mucus protects

Digested microbes are then passed out of the body in faeces.

The stomach produces acid which destroys many of the microbes that enter the body in food and drink.

Blood, Kidneys and Urinary Tract cleanse the body

The macrophages in the blood themselves do die, and are swept up by the kidneys to form urine.

As a consequence, if the kidneys themselves become infected, the urine is often tested for germ infections.

Urine as it flows through the urinary system flushes microbes out of the bladder and urethra.

Microbes that protect against other microbes

The surfaces of the body – the skin, digestive system, and the lining of the nose – are covered by a community of microbes called the normal body flora. They help to protect a host from becoming infected with more harmful micro-organisms by acting as a physical barrier. The normal body flora colonises these linings which reduces the area available for pathogens to attach to and become established. It also means that the harmful microbes have to compete with the normal body flora for nutrients. The average human gut contains around 1 kg of these good bacteria which is equivalent to one bag of sugar.

Antimicrobial Chemicals and Enzymes

The body produces several antimicrobial substances that kill or stop microbes from growing. For example the enzymes in tears and saliva break down bacteria.

Macrophages eat dead tissues and microbes

A further line of defense is to destroy invading organisms.

Phagocytes are a type of white blood cell that move by amoeboid action. They send out pseudopodia which allows them to surround invading microbes and engulf them. Phagocytes release digestive enzymes which break down the trapped microbes before they can do any harm. This process is called phagocytosis.

A macrophage which is a type of phagocyte engulfing a bacterium.

Cyber Security - Antibody and Antigens


Memory Cells

Microbes have Antigens ID microbes - on membranes - recognizable as keys

Antigens are proteins that are found on the surface of the pathogen. Antigens are unique to that pathogen. The whooping cough bacterium, for example, will have different antigens on its surface from the TB bacterium.

Microbe antigens grab recepters on target sites

When an antigen enters the body, the immune system produces antibodies against it. Antibodies are always Y-shaped. It is like a battle with the army (antibody) fighting off the invader (antigen). A type of white blood cell called a lymphocyte recognizes the antigen as being foreign and produces antibodies that are specific to that antigen. Each antibody has a unique binding site shape which locks onto the specific shape of the antigen. The antibodies destroy the antigen (pathogen) which is then engulfed and digested by macrophages.

White blood cells can also produce chemicals called antitoxins which destroy the toxins (poisons) some bacteria produce when they have invaded the body. Tetanus, diphtheria and scarlet fever are all diseases where the bacteria secrete toxins.

  • For example the Covid-19 virus has a STRONG affinity to use "spring loaded" protein sites to
    • Grab spiked hook on ACE receptors on inside of cells on surface of lungs
    • Then physically pull the virus CLOSER to the target cell site
    • This then triggers the cell to cleave open its own membrane - creating an opening
    • The microbe similarly opens up itself to cleave open and insert its DNA/RNA into human cell

Antibodies are proteins that bind on the antigens

Antibody–antigen complex works as the forked end of an antibody recognises and binds to the antigen on the surface of the pathogen.

Macrophages guided by Antibodies to collect marked "garbage"

Normal Immune systems DON'T Attack own host body

Inflammation byproducts trigger Immune system to go hyper!

Normal Immune systems shut off

Once the invading microbes have been destroyed the immune response winds down.

Once a person has had a disease they don’t normally catch it again because the body produces memory cells that are specific to that antigen. The memory cells remember the microbe which caused the disease and rapidly make the correct antibody if the body is exposed to infection again. The pathogen is quickly destroyed preventing symptoms of the disease occuring.

Auto-immune systems - can attack

own cells

Microbes Mutation

Microbes Mutate Relatively Rapidly

Microbes Exchange proteins, DNA and RNA between each other


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