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Microenemies: who and what makes us sick?

  • When another organism enters our bodies and causes damage it leads to an infection
  • The organisms that cause an infection are called pathogens, or infectious agents
  • There are several major types of pathogens:
    • Bacteria – prokaryotic organisms that can produce dangerous toxins and cause inflammation
    • Viruses – have a DNA or RNA packed inside a protein capsid. They invade cells and subvert them so they would help produce new virions
    • Fungi – can cause skin infections and severe lung infections in the people with immunity problems

Sometimes, our body fails us. All of us has experienced being ill: having a cold, getting food poisoning or contracting chickenpox. If we investigate the cause of those illnesses, we would find that they were started because a certain type of organism got inside our bodies and caused damage. These foreign organisms release toxins, take up our nutrients and kill our cells. As a result – we feel ill and have various symptoms: cough, headache or fever. Some of the illnesses caused by pathogens are very easy to treat. Some aren’t. But to understand how to fight various illnesses and find new ways to fight old foes, we first need to understand who is our enemy in the first place?

Viruses: the enemy within

We have encountered viruses much more often then we think. They are a common cause of colds. And each year, the whole world tries to stave of a next influenza epidemic. Influenza virus is one of the most famous ones. It is highly changeable and adaptable, so there is not universal vaccine against it. But most flu cases are non-lethal. On the other hand, there are very severe viral diseases – viral hepatitis or HIV. And to treat them is a great challenge. To understand why, we need to see how viruses function.

It is hard to call viruses “organisms”. Scientists are not sure if they even can be considered alive. They do not have membranes or organelles like cells do. Instead, they are composed of two types of molecules: a nucleic acid (RNA or DNA) and proteins. The nucleic acid is the genetic material of the virus – it holds the information about the viral proteins. The proteins form a protective capsid around the nucleic acid. The capsid is attached to the tail with hook-like fibers – they help with attaching to the plasma membranes of the cells. If the virus is not located inside a cell, it is does not look or act alive. Some viruses cannot even survive in the open air environment. But once a virus finds itself a cell – the situation completely changes.

Simple virus diagram

When a virus encounters a cell it can invade, it uses the fibers of its tail to attach to the membrane. Then the genetic material of the virus is injected into the cell. The viral nucleic acid heads straight for the genome and hacks the “headquarters”. The cell stops making proteins it needs and begin making viral nucleic acids and proteins instead. It also produces the proteins that help in the viral packaging. After a while, the cell is packed with multiple virus particles. When the cell can’t make any more viruses, it bursts and releases its inhabitants. Then they can infect and kill more cells nearby.

Viruses are very hard to treat, as they are hidden inside the cells. Antibiotics that kill bacteria don’t work on them. Viruses are also very changeable because they mutate easily. The typical example is flu virus – we have a new strain every year! And there is a constant struggle to produce vaccines against a novel virus strain in time. Thankfully, common viral diseases such as colds can be overcome by our own immune system as a rule. In case of HIV and hepatitis C they situation is worse. These viruses can damage the immune system itself, so it is much harder to fight them.

The best way to fight against common viral diseases is vaccination. Vaccines usually contain parts of viruses, or there is a culture of cells that have viruses inside. The idea behind vaccines is that they mimic the real viral invasion. Our immune system familiarizes itself with all the information about the viruses, and if we encounter the virus in real life – there are immune cells to fight it. Some viral diseases are especially dangerous for young children – such as measles, rubella and poliomyelitis. Their immune systems are not developed yet, and they are especially vulnerable. That is why it is important to vaccinate them,


Bacteria are the most well known pathogens. Food poisonings, pneumonia and wound infections are all caused by bacteria. These microorganisms are very tiny and look completely different from our own cells.

Bacteria belong to a completely another Kingdom of life – Prokaryota. Prokaryots also have cells that possess both a cellular membrane and sometimes also a robust protective cell wall. But they do not have a nucleus. They do not have any usual organelles that are made from membranes that are present in eukaryotic cells. DNA of prokaryots is unprotected. It forms a ring and is located in the cytoplasm directly.

An example of a bacterial cell

Bacteria can have many different forms: round spheres– cocci, comma – shapes – vibrions, spirals – spirilli. The picture above resembles bacilli – rod-shaped bacteria. All of these forms can live separately or form different kinds of colonies.

Most common shapes of bacteria

 Unlike viruses, bacterial cells are completely independent and are able to make their own proteins and other types of molecules. Sometimes they can produce substances that our own cells are unable to. And they can live in our intestines and help us digest food, providing us with important nutrients. But some bacteria are quite dangerous – they take our own nutrients, and produce toxins that damage our organs. That leads to illnesses. Some of those illnesses are treatable by antibiotics, like penicillin. But as bacteria evolve, and antibiotics are overused, there is a real chance that in the near future most common bacteria would be resistant to antibiotics.


When we think about fungi, we imagine something like this:

A mushroom

But this mushroom is not an entire fungus. The scientists call this a fungal body. It is used by fungi to spread spores, from which new fungi would be able to grow somewhere. But the main fungus is located underneath, in the ground:

An underground mycelium of a mushroom

This is a mycelium. It is formed by multiple fiber-like structures called hyphae (singular: hyphae).

These are the hyphae of the Penicillium – the source of the first ever antibiotic

Hyphae consist of multiple cells that are all surrounded by a cell wall made of chitin. Hyphae grow like roots, from the tip. And they are similar to roots in another aspect – they also absorb nutrients from the environment. But unlike plants, their main food is not water with minerals, but organic material. They usually live on dead plant material or absorb pieces of dead animal and plant bodies from the soil. Fungi can also behave like parasites and infect both animals and plants. In the living hosts, they behave the same way as they do in the soil – their hyphae grow and suck up nutrients from the cells and tissues.

The most common fungal diseases affect the skin. It is easy to contract them in the pool, for example. Such diseases usually affect the foot or the nails – athlete’s foot, for example. But for people that have very low immunity – for example, cancer patients, elderly or HIV-infected – fungal diseases are extremely dangerous. One of such diseases is fungal pneumonia, caused by Aspergillus or Pneumocystis fungi.

Another danger of fungi is that they can contain powerful toxins and can cause severe poisonings. Some fungi are extremely lethal, like fungi of the genus Amanita, like the death cap mushroom below:

Amanita phalloides – this mushroom reportedly tastes good. But it can easily be your last meal

And there are more!

Most of the organisms we have listed here are relatively simply built. But there are also multicellular organisms that can also cause diseases, such as parasitic worms. Another dangerous group are parasitic protists – organisms that have only one cell, but, unlike bacteria, have a nucleus and other organelles. One of the most dangerous protists of this kind is Plasmodium vivax, the organism that causes malaria.

Living as a parasite is very convenient. You do not have to hunt and run around to get food. That is why there are many different pathogens and parasites in the world around us. Thankfully, remembering the rules of hygiene, living and active an healthy life, as well as minding the proper use of medicines can help us not become their unwilling hosts.


Links and further reading:  

[1.] – virus diagram

[2.] – a diagram of a bacterial cell

[3.] – bacterial cells’ shapes

[4.] – a mushroom photo

[5.] – mycelium photo

[6.] – hyphae of the Penicillium

[7.] – A. phalloides photo

[8.] – Online Microbiology Notes