What Causes Non-infectious Disease?

In 1900 a Sydney family that lost a child to diphtheria feared for their other children, because the disease could spread through the household. In 2022 a Sydney family whose parent died of a heart attack had no reason to fear catching the same illness. The ABS 2023 Causes of Death report quantifies the shift: by 2022, 89% of Australian deaths came from non-infectious causes.

An infectious disease is caused by a pathogen, a bacterium, virus, fungus or parasite, that can be transmitted between hosts through contact, droplets, water, food or vectors. A non-infectious disease cannot be transmitted via a pathogen. It arises from internal factors such as gene variants or cellular malfunction, or from interactions with the environment and nutrition.

The dividing criterion is transmissibility, not severity, treatability or genetics. Cystic fibrosis and Huntington's disease are genetic and still non-infectious, because no pathogen passes them between people. Writing that "non-infectious means not genetic" is one of the most common errors in this module.

Infectious triggers of non-infectious disease

Some non-infectious diseases have infectious triggers, and exams test the distinction precisely. Persistent human papillomavirus (HPV) infection can lead to cervical cancer, and Helicobacter pylori infection can lead to gastric cancer. In both cases the pathogen is a risk factor, but the resulting cancer is non-infectious: you cannot catch cancer from a cancer patient.

Cancer is uncontrolled cell division caused by accumulated changes in cell-cycle control. Genetic susceptibility and environmental exposure can both contribute.

The grid above names the categories and gives one example of each. This card adds the mechanism inside each category, because a classification only earns marks when you can state how the cause produces the disease at the level of cells, proteins and DNA. Every example below is one you can quote in an exam.

Genetic: a DNA variant changes a protein

In cystic fibrosis, a CFTR variant (commonly F508del) makes the chloride channel protein misfold, so it is degraded before reaching the cell membrane. Without functional chloride channels, water cannot follow salt into the airways, mucus becomes thick and sticky, and chronic lung infection follows. Huntington's disease comes from a CAG repeat expansion, and PKU from a variant in the phenylalanine hydroxylase gene.

Environmental: an exposure damages cells

Ultraviolet radiation is absorbed by DNA in skin cells and fuses neighbouring thymine bases into thymine dimers. If repair fails, mutations accumulate in genes that control the cell cycle, such as CDKN2A and BRAF, and a melanocyte can begin uncontrolled division: melanoma. Inhaled asbestos fibres scar lung-lining tissue and cause mesothelioma; tobacco smoke carcinogens drive lung cancer and COPD.

Nutritional: deficiency, excess or imbalance

Deficiency deprives cells of raw materials. Too little iron means less haemoglobin per red blood cell and anaemia; too little vitamin D reduces calcium absorption and weakens bone (rickets, osteoporosis); too little iodine limits thyroxine production, so the thyroid enlarges into a goitre. Excess also harms: chronic overconsumption of refined carbohydrate and saturated fat drives insulin resistance and type 2 diabetes.

Cancer: its own category regardless of trigger

Cancer is uncontrolled cell division caused by accumulated faults in cell-cycle control, in checkpoint genes, tumour suppressor genes or proto-oncogenes. The trigger can be genetic (BRCA1/2), environmental (UV, tobacco) or infectious (HPV), but NESA treats cancer separately because the mechanism cuts across every other category. Name the trigger after naming the category.

To show a disease is a growing problem you represent population data over time. Three measures describe that data, and each answers a different question.

These measures reveal a long-term pattern. A century ago most Australians died of infectious disease; today the great majority die of non-infectious disease. Public-health data (AIHW, ABS) name this shift the epidemiological transition: as vaccination, antibiotics and sanitation cut infectious deaths and life expectancy rose, more people live long enough to develop cardiovascular disease, cancer and dementia, so non-infectious disease now dominates mortality.

Reading the data this way also explains why type 2 diabetes is multifactorial. Genetic susceptibility can affect how the body controls glucose, while diet, physical activity, sleep, medicines and social conditions can change risk. No single factor explains every case.

A risk factor is any characteristic, behaviour or exposure that increases the statistical probability of developing a disease. Risk factors are found through epidemiological studies, which show association rather than proof of causation. For cardiovascular disease the list includes family history, smoking, a high-fat diet, physical inactivity, hypertension, obesity, age and type 2 diabetes.

Risk operates at population level, not as individual destiny. A heavy smoker may never develop lung cancer, while a non-smoker may develop it through radon exposure, asbestos, genetic susceptibility or random mutation. Having a risk factor raises the probability; it neither guarantees the disease, nor does its absence guarantee health.

Why multifactorial is the norm

Very few non-infectious diseases are single-factor. Even cystic fibrosis, the textbook genetic disease, varies in severity with respiratory infections, air quality and nutrition. Type 2 diabetes needs both genetic susceptibility (some groups, including Aboriginal and Torres Strait Islander peoples, carry a 3 to 4 times higher baseline risk) and environmental or nutritional triggers such as obesity and inactivity.

Factors also interact. A BRCA1 variant raises breast cancer risk substantially, but the risk is not 100%, and lifestyle factors shift the expressed risk up or down. Think of genetic predisposition as setting the baseline probability and environmental and nutritional factors as moving it, which is why identical exposures can produce different outcomes in siblings.