A brief & basic guide to infectious disease

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Unless you have only recently arrived on planet Earth without reading up about the place first, you will surely know that the world is currently experiencing a disease pandemic. The virus called coronavirus, or COVID-19, very suddenly appeared out of the blue and then just as quickly spread around the globe, both directly and indirectly affecting millions and millions of people in every nation on Earth.

The sheer prevalence of this pandemic has profoundly pervaded the everyday lives of billions of people, either directly or indirectly affecting virtually everyone on Earth. Consequently this has sparked an interest in diseases and viruses among many people. I thought it may be handy and of interest to some to write a brief and very basic post about disease.

I should point out I am not an expert in this field, and am only writing very basic information about infectious disease using reliable and reputable sources. I am not am epidemiologist, but we did learn quite a lot about diseases in university, particularly how they affect animals and ecosystems.

I should also point out that this blog post contains information about disease and sickness which those of a sensitive nature may find upsetting or even repulsive. You have been warned.

Now then, let us begin.

What is disease?

A disease is something that affects living organisms and causes them harm, pain, or impairs its normal function in some way. They are usually characterised by particular signs and symptoms.

An infectious disease is a disease caused by microscopic organisms, which can be transmitted from one person to another. There are many many different kinds of infectious disease, each affecting people with different signs and symptoms. Some can be mild, some can be severe, and others can be deadly. Infectious diseases can be transmitted in very different ways too.

A brief history of infectious disease study:

Infectious diseases have been around as long as human beings. In fact, they have been around since the dawn of time. They are caused by microscopic organisms, also known as microbes.

Not all microbes are bad for us, some do no harm to us, and many are actually beneficial to us. But those which do cause living things harm are known as pathogens. They are also informally known as 'germs'.

Most microbes are seriously tiny. They are so tiny that they are not visible with the naked eye. If you look at your hand you won't see them, but there will be millions of microbes on your skin. There are also billions of microbes inside the human body. And whenever you get a cold or sickness bug, it's pathogenic microbes in your systems that are causing it.

Humans have fought an ongoing war with infectious disease all through time, but because microbes are virtually invisible, nobody actually knew they existed until the 17th century when the first microscopes were invented.

Antonie van Leeuwenhoek was a Dutch man who was the first to see microbes. He worked in a linen drapery, and was interested in crafting glass lenses which he would use to closely examine the quality of the threads. He created lenses that were strong enough to make microbes visible, and thus became the first person to see them.

Antonie van Leeuwenhoek and one of his primitive microscopes.

He thought these intriguing creatures he could see were kinds of super tiny animals, and he called them animalcules. He used his microscopes to look at other things, examining human and plant tissue.

Diagrams van Leeuwenhoek drew of the microscopic things he saw, thinking they were tiny animals which he dubbed 'animalcules'.

His work had a colossal impact on science, and laid the groundwork for microbiology. But even though people were now aware microbes existed, they weren't linked with causing disease until the late 19th century.

Pathogens don't just affect humans, but animals and plants as well. In fact some of the first pathogens ever identified were of those that affected crops.

Before linking disease with pathogens, people had a very crude and rudimentary understanding of diseases, such as their causes. For instance people believed for centuries that illness was caused by an imbalance of certain bodily fluids. Subsequently they treated certain diseases with remedies that would likely repulse and horrify you.

But over the decades humans have learnt more and more about infectious diseases. Now there are even whole branches of medicine that focus on the studies of diseases and microbes. As a result the way we treat disease has improved dramatically, and humans have even managed to wipe out some diseases completely.

In order to treat someone for an infectious disease, the first and most important thing to find out is what actually causes it.

Causes of disease:

Identifying the pathogens which cause particular diseases is critical to knowing how to treat it. There are different kinds of microbes which cause disease, including;

  • Bacteria
  • Fungi
  • Protozoa
  • Parasitic animals
  • And viruses
  • They are known as causative agents.

    Naturally different causative agents cause different diseases, and have to be treated in different ways.

    Diseases caused by bacteria

  • Plague
  • Salmonella
  • Tuberculosis
  • Anthrax
  • Cholera
  • Leprosy
  • Scabies
  • Tetanus
  • Scarlett Fever
  • And many more

    Diseases caused by fungi

  • Ringworm
  • Athlete's Foot
  • Yeast infection

    Diseases caused by viruses

  • Measles
  • Mumps
  • Rubella
  • HIV
  • Smallpox
  • Chickenpox
  • Rhinovirus (common cold)
  • Influenza (flu)
  • Ebola
  • Hepatitis
  • Norovirus
  • Polio
  • Rabies
  • Yellow Fever
  • And, of course, Coronavirus

    Bacteria and viruses are the most common causative agents.

  • Bacteria are microscopic single celled organisms. They do not have nuclei. There are many different species of bacteria (estimated to be 10 quintillion, give or take), and they come in different shapes and sizes.

    Not all bacteria are pathogenic, some are useful to us, and some are neither harmful nor useful to us. The pathogenic ones harm you by creating and releasing toxic chemicals in your bodies.

    Viruses are quite different to bacteria. They are not made of cells, but a shell-like structure made of protein called capsids. Inside are short strands of DNA or RNA.

    As small as bacteria are, viruses are even smaller.

    Some idea of scale:

    The average red blood cell is about 0.007mm wide, that's about 1/10th the width of a human hair.

    Bacteria can vary greatly in size, but for example the Salmonella bacterium is 0.001mm wide.

    And the average virus is even smaller, at 0.00003mm wide.

    So to put that into perspective, if we were to enlarge a virus to the size of a human being...

    ...then a red blood cell would be the size of a football stadium!

    Causative agents cause harm in different ways. Pathogenic bacteria infect organisms with harmful toxins, whereas viruses attack cells in the body, and subsume the cell's own DNA in order to reproduce.

    Exactly how the causative agent spreads also varies between diseases:

    Airborne diseases

  • Measles
  • Tuberculosis

    Diseases spread through airborne droplets

  • Mumps
  • Flu
  • Coronavirus

    Diseases spread through fecal matter

  • Polio
  • Salmonella

    Diseases spread through direct transmission of bodily fluids

  • HIV
  • Ebola

    Diseases spread through mosquitos and ticks

  • Malaria
  • Bubonic plague
  • Zika virus

    Treating infectious diseases:

    Understanding what causes the disease, and also how it spreads, are vital to knowing how to treat it.

    We are able to treat certain diseases with antibiotics, which are medicinal chemicals that inhibit or destroy certain microbes. They are often actually created by other microbes. The widely used antibiotic Penicillin comes from a microscopic fungi Penicillium notatum.

    Some Penicillium notatum in a petri dish.

    Antibiotics can be useful in fighting bacteria, however, antibiotics have NO effect on viruses, because viruses do not have metabolic pathways like bacteria.

    Animals have the power to fight pathogens within their own bodies, using their immune systems:

    Human beings produce white blood cells, which can defend the body from harmful pathogens. There are different kinds.

    Different types of white blood cells deal with different types of pathogens.

    White blood cells valiantly defend the body from pathogens. But, being a white blood cell isn't easy. If you are infected with a virus you've never had before, it will take time for your white blood cells to recognise this new virus and be able to defend against it. In some cases you can be sick for just a few days and then recover. In worse cases, you can be sick for many weeks or even longer.

    Pathogens vary greatly in nature and severity. Viruses alone vary greatly in how they affect people. Some can be mild and make you sick for short periods. Others can make you very sick, or even be lethal. Furthermore, some spread much easier than others.

    Those which spread through the air are unsurprisingly the most contagious kinds of viruses. For example the Common Cold virus spreads very easily from person to person through contaminated air, but has a very low death rate. Rabies, on the other hand, does not spread quite as easily, but has a higher death rate.

    And when we are faced with a virus which both spreads easily and has a high death rate, this can lead to an epidemic, or even a pandemic, like the one we are currently experiencing. Covid-19 can spread quite easily from person to person through the air via droplets which are inhaled, much like cold and flu viruses, but it has a higher fatality rate than these other viruses. It has quite rapidly spread to nearly every country in the world, has infected tens of millions of people, and killed almost 2 million as of writing this blogpost. This is obviously a big concern to the human race, and hence why the prevalence of Covid-19 has been declared a global pandemic.

    Epidemics and Pandemics

    If a disease exists permanently in a particular region and manifests at a fairly constant rate of infection it is referred to as ENDEMIC.
    A classic example of an endemic disease is Malaria, which is endemic in many parts of Africa, Asia, Latin America, and the Middle East.

    If a disease affects many people at a given time across one or several communities at a greater rate than typically expected it is called an EPIDEMIC.
    In the last decade alone there have been many epidemics across the world of diseases such as Dengue Fever, Yellow Fever, Cholera, and Ebola which caused an epidemic in Western Africa between 2013-2016.

    And if a disease has shown a sharp increase of rate of infection that is evident across a very large area or even the entire world it is called a PANDEMIC.

    Human beings have experienced many pandemics through history, even within the last 100 years there have been pandemics of flu viruses, Middle East Respiratory Syndrome (MERS), Cholera, and the Zika virus. And now there is the Covid-19 pandemic.

    Did you know that we are not currently living through one pandemic, but two?

    As well as the Covid-19 pandemic, we are still living through another pandemic which started in 1981: The HIV/AIDS Pandemic.

    The Human Immunodeficiency Virus (HIV) is a deadly virus which attacks the immune system, causing a condition called Acquired Immune Deficiency Syndrome (AIDS), which leaves the host vulnerable to other diseases. It doesn't spread through the air like Covid-19, rather through the direct transmission of bodily fluids. It is believed that about 38 million people are currently infected with HIV around the world.

    No diseases are good, and all of the epidemics and pandemics humans have lived through have had devastating effects on people and whole communities. But rarely are pandemics as widespread and deadly as Covid-19. The last pandemic that is comparable to Covid-19 took place almost exactly 100 years earlier, and is known as The Spanish Flu.

    The Spanish Flu:

    The Spanish Flu of 1918 was a deadly form of influenza that caused a global pandemic between 1918 and 1920. It began in March 1918 as the First World War waged on, and lasted almost exactly two years. Within that period it caused more deaths than WW1 itself.

    It came to be known as Spanish Flu as its effects were more widely reported in Spain than other countries in Europe during its first wave. At this time most of Europe was embroiled in WW1, and belligerent countries avoided reporting on the havoc it was wreaking to avoid causing panic. But neutral Spain had more relaxed reporting regulations, and freely published the flu’s effects; one of the first casualties was the then king of Spain. Thus it appeared that Spain had more cases than other countries, but in reality it was doing much the same damage in other countries across Europe, and was spreading around the world quickly.

    The Spanish Flu is estimated to have lead to the deaths of around 50 million people.

    In many ways the Spanish Flu pandemic is similar to the Covid-19 pandemic. The Spanish Flu is just one type of influenza virus, of which there are many other kinds, and Covid-19 is one kind of many kinds (or strains) of coronavirus. Both spread quite easily between people, and both have higher fatality rates than other viruses of their kind.

    How does a pandemic come to an end?

    When a person's immune system is able to identify particular diseases, it can be able to fight them off before they get a chance to take effect on the body. This is called immunity.

    And in a population of people, if lots of them get immunity to a disease, then the disease has a much lesser chance of infecting people. If a critical mass of people get immunity, the disease can effectively disappear. This is called herd immunity.

    A diagram illustrating herd immunity.

    This is how the Spanish flu came to an end. Sadly in the process many millions died.

    What about vaccinations?

    It is possible to become immune to an infectious disease without having to be infected by it first, thanks to vaccinations.

    Vaccinations are a form of inoculation, when you are injected with the parts of a pathogen the immune system recognises as harmful, so that your body is tricked into thinking you have been infected, and responds to it as if it were the actual disease. The parts of a pathogen which the immune system detects and responds to are its antigens, which are molecules on the surfaces of microbes.

    The very word vaccine/vaccination comes from the Latin word 'vacca', which means cow. The first vaccine was developed by Edward Jenner.

    In Edward Jenner's time (the 18th century) people were plagued by a disease called Smallpox. Smallpox is a horrible disease which causes nasty pustules on the skin. There is another version of smallpox which affects cows (called Cowpox). Edward Jenner noticed that milkmaids seemed to not suffer with smallpox, and he realised it was because they were exposed to cowpox from the cows they milked, which gave them immunity. So he conducted tests wherein he inoculated people with cowpox, which gave them immunity to smallpox, and thus he created the first vaccine, named after the cows in question.

    Smallpox was a terrible disease that mankind had been victim to for thousands of years. But thanks to Edward Jenner, it was possible to fight back. Over the decades, thousands of people were immunised against smallpox. Then in 1979 smallpox was declared completely eradicated.

    Vaccines exist for many diseases, including measles, mumps, rubella, diphtheria, tetanus, and others. A few diseases, like smallpox, have even been all but wiped out from existence thanks to the herd immunity granted by vaccines. But for some diseases it isn't so easy.

    Why can't we just do the same for all viruses then? The reason is because viruses mutate.

    Mutation is when the DNA or RNA of an organism changes naturally. This happens in all organisms, and also happens in viruses. And when viruses mutate their antigens change, meaning that the human immune system doesn't recognise the disease and must become immune to it all over again.

    But, crucially, different viruses mutate at different rates. As terrible as smallpox was, it had a very slow mutation rate, meaning it was much easier to immunise people against than other diseases.

    Conversely the influenza virus has a very fast mutation rate, and so it is much harder to vaccinate against. It is possible to give people some immunity against flu, which is why some people have flu vaccinations, but must have them regularly to keep their immune systems abreast of newly mutated strains of the flu virus.
    They are usually reserved for the most vulnerable people such as the elderly, and given in the winter time when people are more likely to catch it.

    Then you have the HIV virus which mutates so fast it is virtually impossible to vaccinate against.

    What about Covid-19? The rise of Covid has been very sudden and sharp, and it is very difficult to study viruses and their mutation rates, but many immunologists across the world are working hard to create vaccines against Covid which will hopefully help combat it and reduce its deadly impact on the human population. The results of their work are anxiously awaited.

    Nonetheless, whether Covid, HIV, or common cold, it is still possible to reduce the impact of diseases by taking proper precautions to avoid them, and by keeping your immune system as strong as possible by keeping healthy.

    Thank you for reading, I hope this blogpost has been at least somewhat informative. We live in unusual and worrying times, so please take care and look forward to happier times ahead.

    Glossary of terms:

    Epidemiology: The study of the distribution and patterns of disease

    Microbe: A microscopic organism

    Pathogen: A microbe which causes disease in other organisms

    Causative agent: A microbe or body which causes an infectious disease

    Bacteria: Single celled organisms which have no nuclei

    Bacterium: Singular of bacteria

    Nucleus: A cell organelle that contains the chromosomes and directs cell activities

    Virus: A causative agent containing DNA or RNA which harms organisms through parasitic DNA replication

    Antigens: Signature molecules made of proteins or lipids on the surfaces of microbes

    White blood cells: Cells of the immune system which combat pathogens

    Endemic: When a disease is prevalent at a constant rate in a given area

    Epidemic: When a disease affects many people at a given time across a large area at a greater rate than usual

    Pandemic: When a sharp increase of a rate of disease affects people across a very large area or even the entire world

    Antibiotic: Chemicals which inhibit or destroy microbes

    Immune system: A system of organs and processes in the body which provides resistance to infection, toxins, and pathogens

    Immunity: When a person's immune system is able to recognise and combat pathogens, reducing or preventing symptoms of sickness

    Herd immunity: When a critical mass of a population is immune

    Inoculation: The exposure of infectious disease to a person in order to trigger an immune response

    Vaccination: The deliberate introduction of pathogenic antigens in order to immunise someone

    Mutation: When the DNA or RNA of an organism changes naturally

    Strain: A variety of a particular pathogen which has mutated DNA

  • +2
    Level 71
    Jan 19, 2021
    I realise that this 'brief' guide is still quite a long one. It just shows that this is a huge and complex topic. I am thinking of doing other posts about historical epidemics and pandemics if people are interested.
    +2
    Level 52
    Jan 19, 2021
    I liked the virus-human size comparison.
    +1
    Level 51
    Jan 19, 2021
    Good to know...
    +1
    Level 54
    Jan 19, 2021
    Interesting blog, in Pakistan COVID-19's situation is very better
    +1
    Level 43
    Jan 20, 2021
    And here in Brazil... maybe the journals are lying. I think this.
    +2
    Level 54
    Jan 21, 2021
    I think In Brazil, Most of COVID patients are the animals of Amazon. Am I right?
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    Level 43
    Jan 21, 2021
    In the Amazon, the situation is very complicated. Idk if you saw in the news, but in Manaus, the situation is of public calamity. There, the hospitals don’t have oxygen. But in Brazil like a general, the COVID-19 is attacking more than other countries. We are in the epicenter of pandemic. My parents got COVID-19 (despite the negative result), and my mom got dengue too. She almost die. I thank God that I can live more time with she.
    +1
    Level 54
    Jan 22, 2021
    I was joking sorry
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    Level 43
    Jan 22, 2021
    Oh, no problem!
    +1
    Level 78
    Jan 20, 2021
    Very interesting and informative
    +1
    Level 77
    Jan 20, 2021
    This is very informative and interesting. Cool blog!
    +1
    Level 53
    Jan 20, 2021
    A very helpful blog, I learnt quite a bit.
    +1
    Level 43
    Jan 20, 2021
    FINALLY! FINALLY! FINALLY A HELVETICABLOG! LOLOLOLOL!

    I was waiting for this moment!!! Very nice blog!

    +1
    Level 43
    Jan 20, 2021
    Your new name will be HelveticaBlog. LOL
    +1
    Level 43
    Jan 20, 2021
    Curiosity: In 1913 Rodrigues Alves was diagnosed with pernicious anemia, taking periods of rest in Guarujá and graduating from his post between 1913 and 1915. Until the end of his life, periods of improvement and relapse of his health state followed.

    At the end of 1917, his health declined, when the presidential elections of the following year were approaching.

    On March 1, 1918 Rodrigues Alves was again elected president of the republic, being elected as his deputy Delfim Moreira.

    In October 1918, close to the presidential inauguration, Rodrigues Alves contracted the Spanish flu, which plagued the country during that period.

    In November, his condition worsened considerably, which led to the suspension of his inauguration, assuming Delfim Moreira as acting vice president. He died in Rio de Janeiro on January 16, 1919.

    +2
    Level 43
    Jan 20, 2021
    AND OBVIOSLY I'M INTERESTED! YOU ARE ONE OF THE BEST BLOGGERS HERE, AND I MISS YOUR INCREDIBLE WORK!
    +1
    Level 55
    Jan 20, 2021
    Brilliant! I love Anatomy and Diseases
    +1
    Level 55
    Jan 20, 2021
    Why do you like diseases bruh
    +1
    Level 43
    Jan 20, 2021
    Probably, he likes to study the diseases, not the diseases.
    +1
    Level 55
    Jan 20, 2021
    To Study them! Who likes disease?
    +1
    Level 43
    Jan 20, 2021
    The Devon?