What Is Disease?

Disease Types

In science, disease is not "feeling unwell". It is a condition that impairs the normal functioning of the body or mind — and the word doing the work in that sentence is functioning.

That matters because feeling unwell and having a disease come apart in both directions. Skip breakfast, stay up until 1 am and sit a maths test, and you will feel terrible without anything having impaired your functioning: rest and a meal restore you, because nothing was actually disrupted. Meanwhile high blood pressure typically feels like nothing at all for years while it damages arteries the whole time. If we defined disease by how bad someone feels, we would have to call the first case a disease and the second case health, which is the wrong way round. Defining it by function gets both right.

The same test settles the difference between disease and injury. A broken arm certainly impairs functioning, but it is a single event whose damage is then repaired — the process is over the moment it happens, and the body's job afterwards is to heal. A disease involves an ongoing process that keeps disrupting function while it continues. This is why we say someone has asthma but broke their arm.

Once you accept that definition, the interesting question becomes what starts and sustains that ongoing process. There are broadly five answers: a pathogen — a bacterium, virus, fungus or protist living in the body; an inherited fault in a person's genes; the cumulative effect of lifestyle, such as diet, exercise and smoking; environmental exposure to something like radiation, asbestos or air pollution; and the immune system attacking the body's own tissue, which is what an autoimmune disease is. Hold on to that list, because the rest of this lesson splits it in two — and the split is not where most people expect.

Picture a Year 9 student in Sydney sneezing on a packed bus on Monday morning. By Friday six of her classmates have runny noses and a temperature. That chain is the hallmark of an infectious disease: one caused by a pathogen. Most infectious diseases can also pass from one organism to another, and those are called communicable diseases.

It is worth asking why "caused by a pathogen" should usually mean "can spread", because the link is not obvious and it is the whole basis of the classification. The reason is that the cause makes more of itself. A rhinovirus in your nose is not a state your body has got into; it is a population of virus particles being copied, and those copies can be carried out of you on a droplet of mucus and into someone else. The cause can travel because there is a physical thing to travel, and more of it than there was. Nothing else on the list of five can do that — you cannot pass someone a copy of your genes' fault or a copy of your smoking history.

Two cautions, because this is where the words get slippery. First, not every pathogen is alive. Bacteria, fungi and protists are living organisms; viruses are not usually counted as living, because they cannot copy themselves at all without hijacking one of your cells to do it for them. Replication, not life, is what matters for this classification. Second, "infectious" and "communicable" are not quite the same word. An infectious disease is caused by a pathogen; a communicable one can pass from one person to another. Nearly always both are true at once, which is why the terms get used interchangeably — but tetanus is caused by a bacterium that lives in soil and enters through a wound, and you cannot catch it from the person who has it. Infectious, not communicable.

That also tells you what the spread routes have to be: whatever can physically carry a pathogen from where it is to a new host. Pathogens travel in the air in droplets, in water and food, by direct contact with skin or body fluids, and in vectors — other organisms, usually biting insects, that carry the pathogen from host to host. Influenza and COVID-19 (caused by the SARS-CoV-2 virus) mostly travel by air; tuberculosis, caused by Mycobacterium tuberculosis bacteria, is also airborne but needs much longer contact; malaria, caused by Plasmodium protists, cannot travel between people at all without a mosquito to carry it. Handwashing, isolation and vaccination are three different ways of cutting one of those routes, which is why the route matters as much as the pathogen does.

A non-infectious disease is one with no pathogen behind it, so there is no living cause that could move to a new host. These are also called non-communicable diseases, or NCDs. They divide sensibly by what started the ongoing process instead.

Genetic diseases such as cystic fibrosis, Huntington's disease and sickle cell anaemia begin with a fault in a gene that a person is born with. Cystic fibrosis is a good one to understand rather than memorise: a single faulty gene produces a faulty protein in the cells lining the lungs and gut, so the mucus those cells make is far thicker than it should be, which blocks airways and traps bacteria. One fault, a chain of consequences, a lifelong condition — and no way to pass the condition to a friend, because the only way to receive it is to inherit the gene.

Lifestyle diseases such as cardiovascular disease and type 2 diabetes are the accumulated result of diet, physical activity, smoking and alcohol over years. Environmental diseases such as asbestos-related lung disease, and some cancers, trace back to something in a person's surroundings. And degenerative diseases such as Alzheimer's disease and osteoarthritis come from tissue gradually breaking down faster than the body rebuilds it.

One thing to be careful about here, because it looks like a counter-example and is not. Non-infectious diseases often run in families and in whole communities, which can make them look as though they are spreading. Type 2 diabetes is the standard case: it is far more common in some families than others. But what is being shared is the risk factors — the same genes, the same meals, the same neighbourhood with nowhere safe to walk — and not the disease itself. Nobody caught anything. That distinction is what makes prevention possible: you cannot stop a disease being inherited, but you can change what a community eats and how it moves.

Two things surprise people about this half of the list. The first is that it is much the larger half: the World Health Organization attributes about 74% of all deaths worldwide to non-communicable diseases. The second is the reason that number stays stubbornly high — you cannot vaccinate against a lifestyle or quarantine a gene, so preventing these diseases means changing the conditions people live in, over decades. Lesson 21 comes back to what that actually takes.

Now use the table to settle a case it was not written for. Two people arrive at a Sydney clinic, both with a cough that has lasted three weeks, both losing weight, both exhausted. One has tuberculosis; the other has severe asthma. The symptoms overlap, both conditions are serious, both are in the lungs — and yet one patient's family needs testing and the other's does not. Nothing you can see decides it. Only the cause does: TB is a bacterium that can be breathed out and breathed in, and asthma is an inflammatory response of the person's own airways, which nobody can catch.

So the boundary is the cause, and only the cause. Not how severe it is, not how contagious it feels, not whether it is in your lungs. That sounds obvious until it costs you a mark, because two of the commonest exam traps are diseases whose intuitive class is wrong.

Neither case breaks the rule. Both cases show why the rule has to be about the cause: once you know a pathogen is involved, a completely different set of tools becomes available — antibiotics, vaccines, contact tracing — and those tools work no matter how much the disease resembles something else. The reverse is also true, and it is the point Lesson 21 builds on: for the 74% that are non-communicable, none of those tools is available at all.