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Infection & Immunity. [Part One - The Infection & Infectious Diseases.]

Updated: Nov 26, 2021

Pandemic or not, we all like to be healthy. In the scare of the recent outbreak of the virus, people started to research more health-related articles. Apart from the researchers, scientists, and doctors discussing the current subjects, unfortunately, there is also plenty of fake information circulating on social media networks. Feeling quite confused, I’ve decided to dedicate some time to the subject, and so, I did my research. I’ve tried to compile the information regarding the infection itself, the immune system, and the very commonly searched “immunity boosters”.

The article ended up being quite lengthy, so I’ve decided to split it into two parts: Part 1: The Infection & Infectious Diseases, Part 2: The Immune System and Immunity Boosters,

Disclaimer: I am not a trained medical professional, hence all the information below it’s gathered from different sources (look: References). In case of an existing, or suspected illness I strongly recommend talking to a professional health worker and getting a proper diagnosis and more specific information regarding your condition.

Infection vs Disease

Before we get into details of infectious diseases, I would like to bring up a few definitions, which are quite relevant to the subject.

According to the US National Library of Medicine, National Institute of Health:

“Infection” is the term that defines the entrance and development of an infectious agent in a human or animal body, whether or not it develops into a disease. The detection of this state in which there are no signs of a recognised related disease is called “unapparent infection”. The limits between infection and disease are not always clear and may change with the development of new diagnostic techniques.

“Infectious disease” (or “communicable disease”) is defined as an illness caused by a specific infectious agent or its toxic product that results from transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector or inanimate environment.

“Host” is defined as the simple or complex organism that is the target of an infecting action of a specific infectious agent.


What exactly are Infectious Diseases and what causes them?

Dr. Claire Rostron, Senior Lecturer in Health Sciences at The Open University, explains that infectious diseases are diseases that can be passed between people and sometimes between people and animals by infectious agents known as pathogens. (Greek: pathos - to suffer; genès - to produce). The most commonly known types of pathogens are bacteria and viruses.

→ Bacteria are the most numerous pathogens. In fact, there are more than twice as many different types of bacteria that cause human disease compared to the number of infection-causing viruses. Although they can normally be killed by antibiotic drugs, their impact on human health is likely to increase as many become resistant to antibiotics – the drugs specifically used to control them.

Interesting Fact: Not all bacteria are bad! Most of the bacteria that live in or on the human body are beneficial. The number of ‘friendly’ bacteria living in your gut is around 10 times the number of human cells in your whole body! Commensal bacteria (normal microflora) also have a role in protecting us from some infectious diseases. They occupy habitats in the body that could otherwise be colonized by pathogens such as Candida albicans, the fungus that causes thrush (in the mouth and throat).

→ Humans can also get infected by over 200 viruses. Since viruses are not considered living organisms, they can only reproduce new virus particles or virions by invading a living cell. The genetic material of the virus redirects the genetic material of the host cell and causes it to assemble thousands of new virus particles from the naturally occurring chemicals within the cell. Many viruses eventually kill the host cell they infect, for example when the new virus particles disrupt the cell membrane as they are shed into the surrounding environment, where they can infect new cells and begin the cycle of reproduction all over again

Viruses and bacteria can cause many different types of infectious diseases. You probably have heard names like influenza (flu), meningitis, or smallpox. These are just a few examples. Some of the diseases they cause are never or rarely fatal but nevertheless result in millions of acute illness episodes and chronic disabling conditions.

The causes of infectious diseases are many. Ranging from unsanitary living conditions in poverty-stricken communities to inadequate hygiene, even in developed countries.

Another important factor to consider is the age (especially the infancy period, or the elderly), malnourishment, and other illnesses or medications that can weaken the immune system, making it easier for the infection to establish itself in the body.

The most common examples of human behaviors that can help pathogens to spread are:

- Not washing hands, especially after using the toilet or before preparing food.

- Not washing hands properly.

- Not covering the nose and mouth while sneezing or coughing.

- Spitting in the streets.

- Leaving food leftovers uncovered (it invites flies and other insects to settle on it).

- Improperly cooked food, especially raw meat.


Transmission of Pathogens

Since we are talking about pathogens, it’s important to mention the ways in which they can be transmitted. There are three main ways the transmission can occur: directly between people, indirectly (via air, water, food, or sources in the environment), and via other animals to humans.

  • Direct person-to-person transmission of pathogens:

→ Contagious infection, when a touch, such as a handshake, transfers pathogens to a susceptible person; they may enter the new host through a cut or graze, or be transferred from hand to mouth.

→ Sexually transmitted infection (or STI) is the most common route for the worldwide spread of HIV (the human immunodeficiency virus), which causes AIDS (acquired immune deficiency syndrome).

→ Mother-to-child transmission, when pathogens pass from mother to baby during labor and delivery, or via breast milk.

  • Indirect person-to-person transmission of pathogens:

When the original host sheds pathogens into the air, water, food, or objects in the environment, other individuals can get infected via these indirect routes.

→ Droplet Spread - When the infected person sneezes or coughs produce droplets, which can either be inhaled via another person’s airways or can land on a surface, which then might be touched by the infected individual. If the healthy person, who touched an infected surface will touch their face afterward, the virus can be transmitted via their mouth, nose, ears, or eyes. In the “droplet spread” way of transmission, the virus is only in the air for a short time, and travels only a short distance, before it’s pulled down by gravity.

→ Airborne Infections - This way of transmission is very similar to the previously mentioned "droplet spread", although, it behaves slightly differently. As Dr. Amir Khan, an NHS doctor and a senior university lecturer in the United Kingdom explains in an article for Al Jazeera: When a virus becomes airborne, it is described as an "aerosol". This means the virus remains suspended in the air in very tiny water droplets - smaller than the ones coughed or sneezed - long after larger droplets have fallen to surfaces or been breathed in. These tiny droplets can remain in the air for hours if the conditions are right - reduced airflow, open space, and the right temperature. When we think of airborne viruses, the one that most doctors will quote is measles. When a person infected with measles coughs or sneezes, the virus can remain suspended in the air in tiny particles for two hours, awaiting its next victim who will catch it by breathing it in.

→ Waterborne Infections - are particularly common in impoverished communities, where large numbers of people don’t have access to clean drinking water or safe disposal of sewage. Infected urine and feces from humans and animals can wash into lakes and streams, where people collect the water to drink or bathe themselves. The pathogens present in the water multiply, then causing infection in the people using such water sources.

Interesting fact:

Until the 19th century, most Londoners obtained their water from the polluted rivers that flowed through the capital, where London’s domestic and industrial waste was discharged. Several major causes of death in that period were waterborne diarrhoeal diseases and fevers – principally cholera, dysentery, and typhoid. During the cholera outbreak in Soho, London, in 1854, dr John Snow (considered one of the founders of modern epidemiology), traced the source of the disease outbreak to the public water pump on Broad Street (now Broadwick Street), which was commonly used in the area to collect water. He persuaded the local council to disable the well pump by removing its handle, which helped to end the outbreak.

→ Fecal–Oral Infections - when pathogens from feces enter the mouth (via unclean hands, dirty cooking utensils, or contaminated food), and multiply in the gut, they cause a so-called “fecal-oral infection”. Flies can also transfer pathogens from feces to food via their feet.

Once more, unfortunately, this type of infection is commonly found in impoverished areas of the world, as many people are forced to defecate in the open. The soil, which is contaminated, can get on people’s hands, and then pathogens are easily transmitted from hand to mouth, especially when there is no clean water or soap available.

→ Non-living objects in the environment - cups, spoons, door handles, phones, money, and many other objects being routinely touched by people, can also transmit infections. They are collectively known as fomites.

Interesting fact: One type of the fomite can be clothing, which is why hospital doctors now rarely wear neckties or other accessories, that could drape across a patient during a medical examination and pick up an infection that the next patient might acquire.

→ Bloodborne infections - Medical procedures can also transmit infections. Before the transmission of HIV was understood, thousands of infections occurred from HIV-contaminated blood transfusions. Bloodborne pathogens can also spread via shared needles and syringes among people who inject illegal drugs, such as heroin.

  • Animal-to-human transmission of pathogens.

Pathogens are often transmitted from animals to humans ‘accidentally’, for example via infected meat or water contaminated with animal feces.

Zoonoses refer to diseases that are spread from animals to humans. It's possible for humans to catch diseases from a range of vertebrates (the ones that have a backbone, like cows, pigs, bats), and invertebrates (the ones that do not have a backbone, like ticks, mosquitoes, sandflies).

The common example of zoonosis can be influenza that originated in pigs (swine flu), or poultry (bird flu). Some of the viruses may be transmitted during the slaughter or handling of livestock. They can also pose a much greater health risk than the airborne seasonal influenza viruses that commonly circulate in human populations every winter, during the so-called “flu seasons”. The risk of infection with zoonoses increases as the human population continues to grow and people live in greater proximity to animals.

Vector-borne infections are another route of transmission from animals to humans. As mentioned above, they are transmitted by an invertebrate animal. The term “vector” describes the carrier of the disease, e.g. a mosquito. If the vector can be killed, the transmission can be prevented. Common vector-borne diseases are malaria, dengue or Lyme disease, or many others, like the bubonic plague (transmitted by rat fleas), typhus (transmitted by ticks), and yellow fever (transmitted by mosquitoes).

Interesting fact:

Some airplane cabins are sprayed with insecticide before the take-off from a country where vector-borne infections are common. The spray is meant to kill any insects that may have got onto the plane in the clothing or luggage of passengers.


Symptoms and signs, what’s the difference?

Both symptoms and signs are usually considered while making a diagnosis, so it’s important to know the difference in order to give enough information to the health worker. Helping them to gather as much information as possible can allow a better understanding of what we are feeling, which in turn will allow them to identify the underlying cause of the illness and make a diagnosis.

Symptoms are sensations in the body that only we ourselves can feel, and they are not visible to others, including doctors. Only the sufferer can experience symptoms like headache, stomach ache, or nausea, making it impossible to observe or verify.

Signs, on the other hand, are visible, like runny nose or cough.


Acute and chronic conditions

If you’re a fitness enthusiast who got injured at least once, I’m sure you’ve heard the terms “acute” and “chronic”. They can refer to other conditions, like illnesses and diseases as well.

An acute condition means “fast” and “short-term”, but does not necessarily mean “serious”. During an acute infection, the patient develops symptoms rapidly, and within a few days or weeks reaches the “peak”. If the infection is mild, it might resolve in a short time, sometimes without any treatment. In the worst cases, the infected patient might die.

A chronic condition, on the contrary, develops slowly. It may take months or years to reach its most severe period. If left untreated, chronic diseases might very often get worse, and some conditions may result in permanent disability or loss of life.


It's not over yet. The Emerging Infectious Diseases (EID)

Infectious diseases are still very much with us in the 21st century. New infectious diseases are emerging at an accelerating rate and pathogenic bacteria are developing resistance to antibiotics, so in some ways, the threat is increasing, not decreasing.

Emerging infectious diseases (EIDs) is the collective term for a group of conditions that pose new threats to human health. EIDs can be distinguished into three types:

  • New infectious diseases are caused by previously unknown pathogens.

Many in this category are caused by zoonotic viruses. The best known ‘new’ infectious disease is AIDS, but the virus that causes AIDS (HIV) may have originated in monkeys. SARS (Severe Acute Respiratory Syndrome, which may have originated in poultry, MERS (Middle East Respiratory Syndrome), which may have originated in camels, and zoonotic strains of influenza (swine flu and bird flu) are other few which have been identified.

  • Infectious diseases that have spread far outside their original range:

One example is the Ebola virus disease (EVD), a ‘hemorrhagic’ fever, which means it causes severe internal bleeding, among other symptoms.

  • Previously declining infectious diseases that have resurged:

Tuberculosis and some other infections caused by bacteria are a growing health concern because the causative pathogens are becoming increasingly resistant to antibiotics, which previously treated them successfully. So-called ‘hospital superbugs’ are bacteria that have developed resistance to antibiotics in health care facilities, where antibiotics are heavily prescribed.

In the upcoming second part of the article, we will ask questions about the Immune System, Immunity, and the commonly known "boosters" which help to protect us from evil bacteria and viruses.



Barreto, M. L., Teixeira, M. G., & Carmo, E. H. (2006, March). Infectious diseases epidemiology. Retrieved from

Richard L. Riley, M.D. (1974, September 01). Airborne infection. The American Journal of Medicine. Retrieved from

Infection and immunity. (n.d.). Retrieved from

Khan, A. (2020, March 24). Doctors Note: Can coronavirus spread through the air? Retrieved from

Coronavirus: The Lowdown. (2020, March 16). Retrieved from


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