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Infectious Disease

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Biology

Disease is a difficult concept to define. A general definition may be any illness/disorder of the mind or body that results in poor health in an organism, characterised by particular physical/mental manifestations or symptoms. There are many diseases, including hereditary, mental, long-term degenerative and deficiency diseases, but these are all non-infectious diseases. A non-infectious disease can’t be passed on simply through contact with an ill organism. However, there are also various infectious diseases.

What are Infectious Diseases?

Infectious diseases are caused by pathogens, which are microorganisms capable of causing and spreading a disease characterised by a particular set of symptoms. Some contagious diseases affect us for long periods, and others, like the common cold, are much shorter-lived. There are various infectious diseases, each caused by a different pathogen, including different viruses like SARS-CoV-2 that causes COVID-19, or bacteria like Mycobacterium tuberculosis that causes tuberculosis (TB).

Infectious diseases, also known as communicable diseases, are illnesses caused by pathogens that can spread between organisms and infect people, animals or plants simply through exposure to the pathogen or other infected organism.

How are Infectious Diseases spread?

Infectious diseases can be transmitted from infected people to uninfected people simply by direct contact with an ill person. In these cases, the pathogen can’t survive outside the human body, so it must spread via its host.

In other situations, a person can become infected by coming into contact with the pathogen present in fomites like water, animals, faeces, and human food. The pathogen can survive in these elements resulting in the infection being transmitted indirectly between people. Vectors are also tools of transmission. For example, malaria is transmitted to humans via Anopheles mosquito vectors.

A host is an organism that harbours the pathogenic agent.

A fomite is an inanimate component that can transmit the disease when exposed to the pathogen.

A vector is a living organism that transmits the pathogen from one host to another.

Communicable diseases can also affect animals and plants. In animals, any infectious disease is most commonly passed between organisms of the same species, although it can jump species. The method of the passage of a pathogen between hosts is called its transmission cycle.

Some people can become infected with a pathogen but not develop an illness or symptoms. They may even be able to infect and cause illness in others despite not knowing they’re infected in the first place! When this happens, they’re designated an asymptomatic carrier.

Pathogens have different ways to infect and cause illness in an organism while ensuring it spreads among a population. Our understanding of the physiology behind each disease-causing pathogen and its spreading mechanism is crucial to preventing, treating, and/or curing the resulting infectious disease.

Importance of Infectious Diseases

Even though we have witnessed a remarkable decrease in deaths caused by infectious diseases like HIV/AIDS over the last decades, they remain a serious problem. Infectious diseases, like TB or malaria, still infect and kill many people, particularly children and young adults, posing severe public health crises. This problem is more prominent in low-income countries where health services are much weaker and efficient treatment options lag compared with richer countries.

According to the World Health Organisation (WHO), people in these countries are far more likely to die from these diseases than non-communicable diseases, contrary to what is seen in all other richer countries and regions. In fact, as of 2019 and according to the WHO, malaria, tuberculosis and HIV are still in the top 10 leading causes of death in these countries, even though these illnesses can be treated and most can be prevented and cured.

The most current and obvious example of the influence of infectious diseases on our lives is seen by the devastating impact of the current COVID-19 pandemic. COVID-19 is a viral infectious disease caused by the SARS-CoV-2 virus that, according to the WHO, has caused at least 5.8 million deaths in just over two years. The virus is mainly spread by small liquid particles coming from the mouse or nose of an infected person and leads in most cases to mild or moderate respiratory illness, but can lead to more serious symptoms, especially in non-vaccinated people.

When an infectious disease suddenly starts rapidly affecting many people, it creates an epidemic. When the epidemic spreads across a vast region of the world, infecting millions of people, it becomes a pandemic. COVID-19 evolved into a pandemic in 2020 and has infected over 400 million people since, according to the WHO. Many experts believe that COVID-19 will become endemic, meaning a disease that is always in the population but at a much lower and controlled level. Other infectious diseases are endemic to a particular region of the world, like malaria, endemic to tropical and subtropical areas.

Pre-COVID-19 data showed that infectious diseases caused approximately 13 million deaths each year, accounting for around 13% of all deaths, not including the deaths of other organisms like animals and plants that these pathogens also affect. The effects of the COVID-19 pandemic on communicable disease death tolls certainly increased this number in the last few years; however, it appears to be subsiding as vaccinations and new treatments become available.

Vaccinations are one of the primary tools we have to handle and stop infectious diseases by helping to break the transmission cycle of the disease-causing pathogens. Vaccines prime our immune system, so it becomes easier for it to destroy and prevent the spread of an infecting pathogen.

Types of Infectious Diseases

Infectious diseases are divided into four main groups according to the type of pathogen responsible:

  • Bacteria - prokaryotic organisms that cause diseases like TB and cholera and are usually divided into Gram-positive bacteria and Gram-negative bacteria.
  • Viruses are non-living parasitic entities with their own genetic material that require a host to replicate. All viruses are pathogens and cause various viral infections, like COVID-19 or HIV.
  • Protoctista – eukaryotic parasitic organisms that can affect humans, animals or plants, causing infections, most notably malaria.
  • Fungi – eukaryotic organisms that cause fungal infectious diseases most often affect plants.

After infecting an organism, all these pathogens either damage the host’s tissue directly in different ways and/or produce by-product toxins of their metabolism that can also damage their host. The type of damage in combination with how the body responds to it creates the symptoms of the respective infectious disease.

Infectious Diseases examples

There are a variety of infectious diseases of importance still affecting us today, some of which have already been mentioned before, like COVID-19, cholera, TB, HIV/AIDS. Other famous infectious diseases include measles or smallpox.

Smallpox is one of the few infectious diseases that have been completely eradicated in one of the most incredible medical success stories of the past century. After a successful vaccination and surveillance program in the 60s run by the WHO, the last case of this highly infectious and lethal disease caused by the variola virus was detected in 1977 in Somalia. The WHO declared its official eradication in 1980.

The most concerning infectious diseases today are bacterial or viral, except for malaria, which is caused by four different species of the protoctist Plasmodium. Malaria still constitutes a substantial threat to human health, with 40% of the world’s population living in areas at risk of contracting the disease, despite successful efforts to decrease its mortality rate by about 25% across the world.

Infectious Viral Diseases

HIV/AIDS and COVID-19 are perhaps two of the most known viral infectious diseases today. Acquired immune deficiency syndrome or AIDS is caused by the retrovirus Human Immunodeficiency Virus (HIV) and was officially recognised in 1981. Official data by the WHO suggests that at least 25 million people had died from this disease by 2010. People are still dying today, especially in low-income countries in sub-Saharan Africa, where approximately 70% of deaths from this disease occur.

HIV remains one of the most elusive infectious diseases to treat. As of 2022, there is still no cure for AIDS or vaccination against HIV, but treatment options have been very successful at increasing life expectancy and the onset of AIDS.

HIV attacks and destroys our body’s immune system, helper T lymphocytes, which are involved in responding to infections. When these cells are damaged, and our immune system is compromised, it gives rise to opportunistic infections. AIDS is a collection of these diseases brought about by the immunodeficiency caused by HIV.

To learn more about how HIV works, check out our article about it by clicking here.

Infectious Bacterial Diseases

Cholera and TB are two relevant examples of infectious bacterial diseases. Cholera is caused by the pathogenic bacteria Vibrio cholerae and is transmitted through water and food fomites. Cholera is almost exclusive to developing countries where access to proper sanitation and uncontaminated food is problematic. If left untreated, it can be fatal, although death is now entirely avoidable with current treatment options available. Despite this, thousands of people still die every year unnecessarily because of our inability to prevent and treat this illness in developing countries.

TB is another infectious bacterial disease still causing many deaths. TB is caused by the Mycobacterium tuberculosis or the Mycobacterium bovis bacteria that primarily target human cells in the lungs. TB is a serious condition leading to the death of millions of people every year and is often the first opportunistic infection that HIV-positive people suffer. As such, the HIV pandemic has been accompanied by a TB pandemic. Successful antibiotic treatments exist. However, antibiotic-resistant bacteria are becoming a problem. As antibiotic resistance spreads, it brings a re-emergence of this disease that was once thought to be nearly eradicated.

To learn more about how antibiotic-resistant bacteria appear, check out our article on Antibiotics.

Infectious Disease - Key takeaways

  • Infectious diseases are illnesses caused by pathogens that can spread between organisms and infect people, animals, or plants simply through exposure to the pathogen or other infected organisms.
  • Infectious diseases are divided into four main groups according to the pathogen responsible: bacteria, viruses, Protoctista and fungi.
  • There are a variety of infectious diseases of importance still affecting us today, particularly viral infectious diseases such as COVID-19 or HIV/AIDS, as well as bacterial infectious diseases like cholera or TB.

Infectious Disease

Infectious diseases are illnesses caused by pathogens that can spread between organisms and infect people, animals, or plants simply through exposure to the pathogen or other infected organism.

Infectious diseases are divided according to the disease-causing pathogen: 


  • Bacteria
  • Viruses
  • Fungi
  • Protozoa/Protoctista

Tuberculosis (TB), COVID-19, cholera, Malaria, HIV/AIDS.

Infectious diseases are caused by pathogens, which are infective agents of disease.

Contact with an infected organism or direct contact with the pathogen.

Final Infectious Disease Quiz

Question

What is a pathogen’s transmission cycle? 

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Answer

The way the pathogen passes/transmits between hosts.

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Question

Smallpox is a bacterial infectious disease. True or False

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Answer

False 

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Why does HIV/AIDS enable other opportunistic infections?

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Answer

HIV targets immune system cells (T cells) making it more difficult for it to protect us from other pathogens.

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Why is the 2020 global COVID-19 outbreak a pandemic?


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Answer

A pandemic (like COVID-19) affects a large number of people all over the world.

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Which type of pathogen causes cholera?

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Answer

Bacteria

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An asymptomatic carrier is someone infected with a pathogen but that doesn’t develop ________.

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Answer

symptoms/illness

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Malaria is caused by a fungi pathogen. True or False

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Answer

False 

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What do the common cold, the flu, and COVID-19 all have in common?

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Answer

They’re all viral infectious diseases

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Where do infectious diseases like cholera or HIV  kill more people in the world?

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Answer

Low-income countries

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What is one of the main emerging challenges to treating TB?


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Answer

The appearance of antibiotic-resistant TB.

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When a pathogen spreads indirectly between humans it requires _____.

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Answer

Vectors

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What is one of the major steps we can do to help us fight infectious diseases?

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Answer

Vaccines/vaccinations 

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TB can be caused by two different bacteria. True or False

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True

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Which of these infectious diseases does not have a vaccine available?

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HIV/AIDS

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All infectious diseases always affect us for long periods of time. True or False

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False

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What kind of pathogen infection results in cholera?

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Answer

Bacteria (Vibrio Cholerae)

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There is no vaccine for cholera. True or False.

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Answer

False

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Why is cholera more widespread in developing countries?


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Answer

Lack of access to safe drinking water and proper sanitation facilities.

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Cholera is water and food-borne. What does this mean?

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Answer

Cholera bacteria can survive in water and food and be transmitted through the ingestion of contaminated water/food.

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Why is direct person-to-person transmission of cholera unlikely? 

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Answer

Cholera bacteria exits the body of an infected person through its faeces.

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Antibiotics are always recommended for cholera infections. True or False 


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Answer

False

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Symptoms of cholera include 

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Answer

Diarrhoea and vomiting

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What is the main symptom of cholera?

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Diarrhoea

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Why is oral rehydration therapy the main treatment strategy for cholera?

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Answer

It provides the body with the fluids it lost as a result of cholera diarrhoea.

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Which of the following actions should you not do when travelling to an area with a cholera outbreak?


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Answer

Eat shellfish

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How many strains of Vibrio Cholerae are there?

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Answer

There are many different strains but only two can cause cholera.

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What events cause particular concern over possible cholera outbreaks?

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Answer

Natural disasters

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What is oral rehydration therapy?

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Answer

A solution composed of salts and glucose that is either ingested or injected into a patient with cholera.

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What food can concentrate the cholera bacteria very easily?


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Filter-feeder shellfish like oysters or muscles.

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Why is a quick diagnosis of cholera important?

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Answer

In severe cases, people can die within 24hours of cholera symptom onset.

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TB is a viral infectious disease. True or false?

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Answer

False

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What organ do the TB bacteria primarily target?

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Answer

Lungs

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What is the name of the TB vaccine?

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BCG

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What is the main method of transmission of tuberculosis?

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Airborne droplets containing TB germs.

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All people infected with TB bacteria have TB disease. True or false?

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Answer

False

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What kind of drug therapeutics are used to treat TB?

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Antibiotics

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Why is HIV/AIDS a significant risk factor in developing TB disease?

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Answer

It weakens the immune system and its fight against TB bacteria.

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People can become infected with TB by drinking milk. True or false?


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True

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Why is the treatment for TB several months long?

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To ensure all bacteria are killed.

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What is one of the main challenges to the effectiveness of TB therapeutics?

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Answer

Antibiotic resistance

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What are the symptoms of TB disease?

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Racking cough, fever, weight loss, sweating, chest pains, shortness of breath.

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What is MDR-TB?

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Multidrug Resistant Tuberculosis

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A chest X-ray can be used to diagnose TB infection. True or false?


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Answer

False

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What strain of TB can spread from cattle to humans?

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Answer

Mycobacterium bovis

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Which of the following strategies should not be used to tackle TB outbreaks?

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Answer

Widespread BCG vaccination

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Malaria is a bacterial infectious disease. True or False?

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Answer

False

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Malaria is caused by what organism species?

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Answer

Plasmodium

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What is a vector?


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Answer

Vectors are organisms that carry the infectious disease pathogen between people or between animals and humans.

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What cells does the malaria pathogen first attack in humans?


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Answer

Liver cells

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Question

What is cerebral malaria?

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Answer

Malaria condition that affects the brain causing neurological symptoms like seizures, coma and abnormal behaviour.

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