Communicable vs Noncommunicable Disease Overview

A communicable disease is an illness caused by a pathogen that can be passed from one host to another, while a noncommunicable disease (NCD) is not caused by an infection and does not spread between people; examples are influenza and tuberculosis (communicable) and heart disease, cancer and type 2 diabetes (noncommunicable). In brief: communicable diseases are caused by bacteria, viruses, fungi, protists and parasitic worms, and are controlled by hygiene, vaccination and treatment. Noncommunicable diseases arise from genes, lifestyle, age and environment, and are now the leading cause of death worldwide.

Health is more than the absence of disease: the World Health Organization defines it as a state of complete physical, mental and social well-being. This guide defines both groups of disease, explains how infections spread and how diseases are measured, compares communicable and noncommunicable diseases in a table, works through an incidence and prevalence calculation and corrects common mistakes.

What are health and disease?

Disease is a disorder of the structure or function of the body that produces symptoms (what the patient feels) and signs (what an observer can measure). Diseases can be classified by cause: infectious (pathogens), genetic (inherited mutations), degenerative (wear and aging), nutritional (deficiency or excess), environmental (toxins, radiation), autoimmune (the immune system attacks the body), mental and cancers.

What are communicable diseases?

A communicable (infectious) disease is caused by a pathogen, an organism that causes disease. The main groups are:

• Viruses: influenza, measles, HIV/AIDS, COVID-19, hepatitis. Antibiotics do not work on them.
• Bacteria: tuberculosis, cholera, tetanus, strep throat. They are treated with antibiotics.
• Fungi: athlete’s foot and ringworm.
• Protists: malaria (Plasmodium), which is carried by mosquitoes.
• Parasitic worms and arthropods: tapeworms, hookworms, lice and scabies mites.

Some diseases, such as measles, are passed directly between people, and others, such as malaria, need a vector, an organism that carries the pathogen.

How do infections spread?

• Direct contact: touching, kissing, sexual contact, or contact with blood (HIV, hepatitis B).
• Droplets and aerosols: coughing and sneezing (influenza, colds, tuberculosis).
• Contaminated food and water: cholera, typhoid, salmonella.
• Vectors: mosquitoes (malaria, dengue), ticks (Lyme disease) and fleas.
• Surfaces (fomites): objects such as door handles that carry pathogens.
• From animals to humans (zoonoses): rabies, and many new diseases.
• From mother to child: across the placenta, during birth or in milk.

Control measures break the chain: washing hands, clean water and sanitation, cooking food, controlling vectors, quarantine of the sick, condoms and vaccination. The basic reproduction number R₀ is the average number of people that one case infects in a fully susceptible population. If R₀ is greater than 1, the outbreak grows. Measles has an R₀ often estimated at 12 to 18, which is why it needs very high vaccination coverage (about 95 percent) to stop spreading.

What are noncommunicable diseases?

NCDs are long-lasting (chronic), and result from a mix of genetic, physiological, environmental and behavioral factors. The four main groups according to the WHO are cardiovascular diseases, cancers, chronic respiratory diseases (such as asthma and COPD) and diabetes. Others include mental disorders, genetic disorders, allergies and degenerative diseases such as Alzheimer’s disease.

The main risk factors are tobacco use, an unhealthy diet, physical inactivity, harmful use of alcohol, air pollution and aging. Some of these, such as smoking, can be changed; others, such as age and inherited genes, cannot. The WHO estimates that NCDs cause roughly three-quarters of non-pandemic deaths worldwide, and most of the deaths before the age of 70 in low- and middle-income countries are now due to them.

Some diseases do not fit simply into one group: infections can cause noncommunicable disease (human papillomavirus can cause cervical cancer, and Helicobacter pylori can cause stomach ulcers and cancer), and noncommunicable diseases can increase the risk of infection (diabetes).

Communicable and noncommunicable diseases compared

How is disease measured?

• Incidence: the number of new cases in a population in a given period, divided by the number of people at risk (those who do not already have the disease).
• Prevalence: the proportion of the population that has the disease at a given time, including old and new cases.
• Mortality rate: deaths per population in a given time. Case fatality rate: the proportion of cases that die.
• Epidemic: an increase in cases above the expected level in a region. Pandemic: an epidemic that spreads across many countries. Endemic: a disease that is constantly present in an area.

Worked example: incidence and prevalence

A town has 20,000 residents. At the start of a year, 400 people have a chronic disease. During the year, 150 other residents develop the disease. For simplicity, assume that nobody recovers, dies or moves away.

1. Prevalence at the start: 400 ÷ 20,000 = 0.02, or 2.0 percent.
2. People at risk: 20,000 − 400 = 19,600 (those who do not have the disease at the start).
3. Incidence over the year: 150 ÷ 19,600 = 0.00765, or about 7.7 new cases per 1,000 people at risk.
4. Prevalence at the end: (400 + 150) ÷ 20,000 = 550 ÷ 20,000 = 0.0275, or 2.75 percent.

Prevalence rises because new cases are added and no one leaves the group. For a short illness, such as a cold, prevalence is low even when incidence is high, because people recover quickly. For a long illness, such as diabetes, prevalence is high compared with incidence.

How did ideas about disease change?

Hippocrates argued around 400 BCE that disease has natural causes. John Snow traced a cholera outbreak in London in 1854 to a water pump in Broad Street, an early work in epidemiology. Louis Pasteur and Robert Koch established the germ theory in the 1860s to 1880s, and Koch’s postulates were criteria for linking a microbe to a disease. Edward Jenner’s smallpox vaccination (1796) led to the eradication of smallpox, which was declared in 1980. Alexander Fleming discovered penicillin in 1928. In the twentieth century, as infections declined in wealthy countries, noncommunicable diseases became the main burden, and the Framingham Heart Study (started in 1948) and Doll and Hill’s work on smoking and lung cancer (1950s) identified risk factors.

Which mistakes are common?

Mistake 1: Saying that antibiotics cure viral infections

Antibiotics act on bacteria. Viral infections need vaccines or antiviral drugs, or are treated by supportive care.

Mistake 2: Assuming that noncommunicable means not infectious in origin

Some cancers and ulcers have infectious causes, though the disease itself is not passed from person to person.

Mistake 3: Confusing incidence and prevalence

Incidence counts new cases over a period, and prevalence counts all existing cases at one time.

Mistake 4: Assuming that a risk factor guarantees disease

Risk factors raise the probability of disease, but do not make it certain.