What Are Pathogens? Understanding the Germs That Make You Sick

-Pathogens: The Supervillains Inside Your Castle

We learnt that body is like a castle, and our immune system is its team of superheroes, always on patrol to keep everyone safe.

But what are they defending against?

Meet the Pathogens? The Villains of Our Story

Any organism that has the power to make you sick is called a pathogen. Think of them as the supervillains! Some supervillains, like the Influenza Virus (a speedy, invisible villain that causes the flu) or Typhoid Bacillus (a sneaky villain that causes typhoid fever), are always up to no good. If they get into your castle, they'll definitely try to cause trouble and make you sick.

But then there are other supervillains, known as opportunistic pathogens. These are like villains who usually just hang out in your castle, causing no harm. They might even live peacefully in some parts of your body. But if your castle’s superheroes (our immune system) get weak, or if they accidentally end up in the "wrong neighborhood" (a part of your body where they shouldn't be), then they suddenly become dangerous and cause illness. It's like a villain who's normally harmless but turns evil if their power-dampening cuffs come off or if they get a special weapon!

The Four Types of Supervillains

Pathogens come in four main types, each with their own unique powers and weaknesses:

• Bacteria: These are like tiny, single-celled foot soldiers. Some are good (like friendly neighborhood heroes), but many can cause trouble. Think of them like the Goblins– lots of them, and some can cause big problems!

• Viruses: These are even tinier, like invisible shapeshifters. They can't do much on their own, but once they get inside your body's cells, they take over and make copies of themselves, causing a lot of damage. The Influenza Virus is a prime example. Imagine Loki – always trying to get inside and manipulate things from within!

• Fungi: These are organisms like molds and yeasts. Most of the time, they're harmless, but some can cause infections, especially on your skin or in other specific areas. Think of them as creatures from another dimension that might pop up and cause fungal infections.

• Internal Parasites: This is a broad group that includes tiny, single-celled organisms called protozoa and even multi-celled creatures like worms. These villains live inside your body, often feeding off your resources. Imagine them as alien invaders who set up a base inside your castle and start draining its energy.

A Look at the Villains

Let’s take a closer look at these microbes. Here's an Images (taken from The Immune System by Peter Parham, 4th Ed.) showing a few of the real-life villains:

(Images taken from The Immune System-Peter Parham, Fourth Edition)

Image Description:- Many different microorganisms can be human pathogens. (a) Human immunodeficiency virus (HIV), the cause of AIDS. (b) Influenza virus. (c) Staphylococcus aureus, a bacterium that colonizes human skin, is the common cause of pimples and boils, and can also cause food poisoning. (d) Streptococcus pneumoniae, is the major cause of pneumonia and is also a common cause of meningitis in children and the elderly. (e) Salmonella enteritidis, the bacterium that commonly causes food poisoning. (f) Mycobacterium tuberculosis, the bacterium that causes tuberculosis. (g) A human cell (coloured green) containing Listeria monocytogenes (coloured yellow), a bacterium that can contaminate processed food, causing disease (listeriosis) in pregnant women and immunosuppressed individuals. (h) Pneumocystis carinii, an opportunistic fungus that infects patients with acquired immunodeficiency syndrome (AIDS) and other immunosuppressed individuals. The fungal cells (coloured green) are in lung tissue. (i) Epidermophyton floccosum, the fungus that causes ringworm. (j) The fungus Candida albicans, a normal inhabitant of the human body that occasionally causes thrush and more severe systemic infections. (k) Red blood cells and Trypanosoma brucei (coloured orange), a protozoan that causes African sleeping sickness. (l) Schistosoma mansoni, the helminth worm that causes schistosomiasis. The adult intestinal blood fluke forms are shown: the male is thick and bluish, the female thin and white. All the photos are false-coloured electron micrographs, with the exception of (l), which is a light micrograph.

The Ever-Changing Battle: Host vs. Pathogen

Over a very long time, the relationship between a supervillain (pathogen) and Castle (Your Body) changes. Supervillains often evolve special tricks to invade your castle, make lots of copies of themselves, and then escape to infect other rooms (body parts of human system) of the castle.

But here's a secret: a supervillain usually doesn't want to destroy its host castle too quickly! If the castle is completely ruined, the villain loses its home and its source of power (Just like Joker needs batman). So, the really dangerous villains who cause fast and deadly diseases often learn to "play nice" over time, finding a way to live with their hosts without killing them off too quickly.

In the same way, human populations also evolve. We develop a natural "built-in" genetic shield against common supervillains. We also gain lifetime immunity to diseases that are always around, like measles, chickenpox, and malaria. These are called endemic diseases, meaning they're common in a certain population, and most people get exposed to them when they're kids.

Because of this constant back-and-forth between the host (you!) and the pathogen (the villain!), the way infectious diseases act in humans is always changing.

The Flu: A Common, Yet Powerful Foe

Take the Influenza Virus (the flu) as an example. It's a common viral disease, and even though its symptoms can be really tough like getting hit by a Hulk smash of fever, aches, and exhaustion your immune system usually beats it! It's hard to imagine fighting off bad guys when you're feeling so weak from the flu, right?

Yet, for most healthy people in places where the flu is common, it's usually not a big danger. If you're warm, well-fed, and otherwise healthy, you'll usually recover in a couple of weeks. You just trust that your Avengers team (your immune system) will get the job done.

However, when a supervillain is new to the human population, it can be really dangerous and cause a lot of harm. For example, the Corona virus (COVID-19) was a new and terrifying villain, and it caused a very high death rate in those it infected. This is because our Avengers team hasn't encountered this villain before and doesn't know its weaknesses or how to fight it effectively right away.

Your castle is constantly under threat but it’s also brilliantly defended.

Some invaders come in disguise. Some evolve. Some go quiet for years.

But your body is always learning, remembering, and upgrading its defences like a real-life superhero movie that never ends

References:

1. Parham, P., & Parham, P. (2014). The Immune System (4th ed.). W.W. Norton & Company. https://doi.org/10.1201/9781317511571

2. Kuby Immunology Judy A. Owen, Jenni Punt, Sharon Stranford