Infectious Disease

Many organisms live in and on our bodies, causing no harm! However, under certain conditions, these organisms can manifest into disease. 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. A non-infectious disease can’t be passed on simply through contact with an ill organism. However, there are also various infectious diseases that can be passed on!

A description of an infectious disease

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 which causes COVID-19, or bacteria like Mycobacterium tuberculosis which causes tuberculosis (TB).

What causes the spread of infectious diseases?

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.

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.

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.

The 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.

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 countries and regions with stronger health infrastructures. 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.

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 new treatments become available.

The different 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 – 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 but can affect humans, like athlete's foot.

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.

Examples of infectious diseases

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 well-known infectious diseases include measles and smallpox.

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.

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 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 countries with poor health infrastructure.

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.