Easy · Human Biology

What is a disease?

A disease is a condition that has an adverse effect on a person’s health. Everyone on the planet, including you, will have harboured a disease at least once in your lifetime. Diseases are hugely diverse, ranging from the common cold to cancer. Symptoms that present with disease may have very mild effects that can be ignored or treated with over the counter drugs, such as a runny nose or a sore throat. Other symptoms are more severe and may need medical attention, including wheezing and dizziness, or chest pain and difficulty breathing.

Disease can be split into two main flavours; infectious and non-infectious. Here, we will be focusing on infectious diseases. These are caused by tiny germs, known as pathogens, that have entered the body. Pathogens include bacteria, viruses, and fungi.

Figure 1. A microscopic image of Salmonella, a bacterium that causes food poisoning if ingested.

Bacteria are single-celled microorganisms with a simple internal structure. They thrive in a range of environments, from inside your body to between your bedsheets (this may seem gross at first, but read this to give yourself some peace of mind). Salmonella, shown in figure 1, is a bacterium that is spread by the ingestion of contaminated food, and can cause food poisoning and typhoid fever. However, not all bacteria are bad for you. Yoghurts normally contain probiotics, which are good bacteria, that keep the gut healthy by controlling the growth of harmful bacteria.

Viruses are very unique pathogens because they must live inside a host cell, one of your own body’s cells, in order to survive and reproduce. Infected host cells are taken over by the virus, with the virus manipulating the cells functions to carry out its own requirements. The virus causes the host to produce more viral cells. The take-over of the host cell may result in the host cells death. Viruses cannot be treated with antibiotics and instead an antiviral vaccination must be provided; people tend to be vaccinated at a young age, as these jabs tend to provide us with life-long immunity against that virus.

Not what you think – fungi don’t always cause disease! Mushrooms are a fungus, as is the yeast found in marmite and bread.

When you think of fungi, you might think of mushrooms, or maybe mould. Fungi are an incredibly diverse group of organisms, containing somewhere between 1.5 million to 5 million species that can be found just about anywhere. In disease fungal spores either attach onto the skin, causing infections such as athletes foot, or can be breathed in, affecting the lungs.

To cause disease, pathogens must enter the body. Pathogens can enter the body through cuts in the skin, by being breathed in or by being ingested to name a few processes. If the pathogen gets inside the body and survives, it can reproduce and cause disease. The body has a defence system, known as the immune system, that fights off pathogens. However, the immune system cannot always defeat the illness; pathogens are much better at evolving to their environment than the immune system is, giving the pathogen an advantage.
Antibiotics are drugs that can be administered upon a bacterial infection. They can either kill bacteria or suppress bacterial growth enough so that the immune system can rid the body of the infection. However, antibiotics must be used with caution – overuse or misuse of antibiotics causes bacteria to become resistant to the antibiotic, creating strains of antibiotic resistant bacteria. This has major repercussions on the medical world, as without effective antibiotics processes such as surgery will become compromised.

Diseases are caused by pathogens, but not all pathogens cause disease. Some bacteria are good for us, and can even provide health benefits for us. Fungi consists of a whole range of organisms that are useful to us; for example, yeast is a type of fungus that causes bread to rise. Diseases can be eradicated with the body’s own immune system, but some may require extra help in the form of antibiotics or drug therapies. Unfortunately, some diseases are incurable, although there are schemes in place to provide emotional and physical care to people suffering with these types of illness.

4 thoughts on “What is a disease?

  1. Hi Sophie! Great post! I just have one question….you talked about how when the pathogen enters the body it reproduces to become a disease….does our immune system ever recognise the pathogen before it becomes a disease and fights it then? Or does it have to wait until the pathogen becomes a disease to fight it? Is this why maybe some diseases are harder to fight? because it became a strong disease before the immune system noticed? Thanks in advance!


    1. Hi Grace, thank you for your question!
      Within the immune system there are two branches – the innate immune system and the adaptive immune system. The innate immune system is the body’s first line of defense when it comes to the pathogen. Sometimes, cells of the innate immune system recognise and destroy the pathogen before it gets the chance to cause a disease. However, pathogens have developed strategies that allow them to evade the innate immune system, and this is when a disease tends to develop. In this case, the adaptive immune system will kick in to try and eliminate the pathogen. The adaptive immune system is highly specific compared to the innate immune system, and provides us with a memory of the pathogen that caused the disease; this allows the adaptive immune system to recognise that pathogen if it comes into contact with the body, allowing the adaptive immune system to destroy any pathogens before they cause a disease. This is what happens in the case of chickenpox.
      You are right in saying that some diseases are harder to fight if they have had time to develop before being recognised by the immune system. Cancer is a good example of this. In some cases, cancer is only diagnosed in the later stages, where it cannot be treated as effectively. This is because the cancerous cells of the tumour have had more time to grow and replicate, causing the tumour to get bigger. Cancer can also spread across the body if it has not been diagnosed at the early stages. In essence, if cancerous cells or the tumour is not recognised by the immune system, the cancer has an opportunity to grow stronger, making it more difficult to treat.
      I hope this answers your question!


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