A communicable disease is caused by a pathogen — a microorganism such as a bacterium, virus, fungus, or parasite — and can pass from one person to another or from an animal to a person. A non-communicable disease, such as cancer or type 2 diabetes, cannot be passed on; it arises from genetics, lifestyle, or environmental factors.
What is a communicable disease?
A communicable disease (also called an infectious or contagious disease) is caused by a living pathogen that invades a host organism. Pathogens multiply inside the host, causing damage directly or by releasing toxins, which produces the symptoms of disease.
The four main types of pathogen:
| Pathogen type | Example | Disease caused | How it harms the host |
|---|---|---|---|
| Bacterium | Streptococcus | Strep throat | Produces toxins; destroys tissue |
| Virus | Influenza virus | Flu | Invades and destroys host cells to replicate |
| Fungus | Tinea species | Ringworm, athlete's foot | Grows on and digests skin |
| Parasite | Plasmodium | Malaria | Destroys red blood cells; Anopheles mosquito is the vector |
Communicable diseases spread by various routes: airborne droplets (coughing, sneezing), direct contact, contaminated food or water, body fluids, or animal vectors.
What is a non-communicable disease?
A non-communicable disease (NCD) cannot spread from person to person. It develops as a result of a combination of genetic factors, lifestyle choices, and environmental exposures accumulated over time. NCDs are increasingly common in high-income countries as people live longer.
Major types of NCD:
- Cardiovascular disease (heart disease, stroke): linked to high blood pressure, high cholesterol, smoking, lack of exercise.
- Type 2 diabetes: linked to obesity and insufficient physical activity; the body's cells become resistant to insulin.
- Cancer: uncontrolled cell division caused by mutations in DNA; some cancers have lifestyle risk factors (lung cancer and smoking), others have genetic causes.
- Chronic respiratory disease (asthma, COPD): linked to air pollution, smoking, or inherited airway sensitivity.
How do communicable and non-communicable diseases compare?
| Feature | Communicable | Non-communicable |
|---|---|---|
| Caused by | Pathogen (bacterium, virus, fungus, parasite) | Genetics, lifestyle, environment |
| Can it spread person-to-person? | Yes | No |
| Can antibiotics treat it? | Sometimes (bacteria only, not viruses) | No |
| Examples | Flu, TB, cholera, malaria, ringworm | Heart disease, type 2 diabetes, cancer, asthma |
| Main prevention strategy | Vaccines, hygiene, vector control | Healthy lifestyle, screening, avoiding known risk factors |
What are the risk factors for non-communicable disease?
A risk factor is anything that increases the probability of developing a disease. Risk factors do not guarantee disease — they increase the likelihood.
Modifiable risk factors (things people can change):
- Smoking — strongly linked to lung cancer, COPD, and cardiovascular disease.
- Poor diet — high saturated fat increases cholesterol; excess sugar promotes obesity.
- Physical inactivity — raises risk of heart disease, type 2 diabetes, and some cancers.
- Excess alcohol — linked to liver disease and several cancers.
- Obesity — increases risk of type 2 diabetes, heart disease, and certain cancers.
Non-modifiable risk factors (things people cannot change):
- Age — many NCDs become more common with increasing age.
- Sex — some conditions are more prevalent in one sex.
- Genetic inheritance — family history of heart disease or cancer raises personal risk.
How are communicable diseases prevented and treated?
Communicable diseases can be tackled at the individual and population level:
- Vaccination: introduces a weakened or dead form of the pathogen (or its antigens) so the immune system learns to recognise and fight it without causing disease.
- Hygiene: hand-washing, sterilising food preparation surfaces, and sewage treatment remove pathogens before they can spread.
- Vector control: mosquito nets and insecticides reduce malaria transmission by controlling Anopheles mosquito populations.
- Antibiotics: kill or inhibit bacteria inside the body; they have no effect against viruses.
- Quarantine and isolation: keeping infected individuals away from healthy people reduces spread of highly contagious diseases.
Antibiotic resistance is a major global health threat: overuse of antibiotics has allowed some bacteria (such as MRSA) to evolve resistance, making once-treatable infections potentially fatal.
Frequently asked questions
Can a disease be both communicable and non-communicable?
Most diseases are clearly one or the other, but a communicable infection can trigger a non-communicable condition. For example, the human papillomavirus (HPV) is a communicable infection spread by sexual contact, but persistent HPV infection causes mutations that lead to cervical cancer — a non-communicable disease. Similarly, the Helicobacter pylori bacterium increases the risk of stomach ulcers and stomach cancer. The cancer itself is not infectious, but its root cause was an infection.
Why do antibiotics not work against viral diseases such as flu?
Antibiotics target specific structures found in bacterial cells — for example, the bacterial cell wall — that do not exist in viruses or in human cells. Viruses are not cells at all; they are fragments of genetic material enclosed in a protein coat, and they replicate inside the host's own cells. Antibiotics have nothing to attach to and nothing to block in a virus. Antiviral medicines exist for some viruses (such as oseltamivir for flu), but they work by very different mechanisms from antibiotics.
What is the difference between a vector and a pathogen?
A pathogen is the organism that actually causes the disease — for malaria, this is the Plasmodium parasite. A vector is an organism that carries and transmits the pathogen from one host to another without itself being the cause of disease — for malaria, this is the female Anopheles mosquito, which picks up Plasmodium from an infected person's blood and injects it into the next person it bites. Controlling the vector (mosquitoes) is therefore one strategy for fighting the disease even without a fully effective vaccine.
How does herd immunity protect people who cannot be vaccinated?
Herd immunity occurs when enough of a population is immune to a disease (through vaccination or previous infection) that the pathogen cannot spread easily — even unvaccinated individuals are protected because the chain of transmission is broken. The proportion needed for herd immunity varies by disease: measles, which is highly contagious, requires about 95% of the population to be immune. People who cannot be vaccinated — newborns, immunocompromised patients, those with certain allergies — depend on herd immunity for protection.
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