Pathogens, Disease Transmission, and Vectors

A pathogen is a microorganism that causes disease, and infectious diseases spread from person to person or from the environment through direct transmission (contact with an infected host) or indirect transmission (through air, water, food, objects or vectors). In brief: pathogens include bacteria, viruses, fungi, protozoa and parasitic worms. A vector is an organism, usually an insect such as the Anopheles mosquito, that carries a pathogen from one host to another. Breaking the chain of transmission is the main way to control infectious diseases.

Infectious diseases still cause millions of deaths each year, especially in low-income countries, so understanding how they spread is the basis of prevention. This guide explains what pathogens are, how disease spreads in direct and indirect ways, the role of vectors, how to break the chain of infection, works through an example with malaria and cholera and corrects common mistakes.

What are pathogens and infectious diseases?

A pathogen is any organism or virus that can cause disease in a host. A disease is a disorder that affects the function of the body, and an infectious (communicable) disease is one caused by a pathogen that can be passed from one host to another. Non-infectious diseases, such as heart disease and diabetes, are not caused by pathogens and do not spread this way.

• Bacteria: release toxins or damage tissue. Examples: tuberculosis, cholera, salmonellosis, tetanus.
• Viruses: reproduce inside cells and destroy them. Examples: influenza, measles, HIV/AIDS, COVID-19.
• Fungi: examples: athlete’s foot, ringworm, thrush.
• Protozoa: examples: malaria, amoebic dysentery, sleeping sickness.
• Parasitic worms and arthropods: examples: tapeworms, roundworms, head lice, scabies mites.

To cause disease, a pathogen must enter the body, avoid defenses, multiply and cause harm. The time between infection and the appearance of symptoms is the incubation period, which varies from hours (food poisoning) to years (HIV).

How does direct transmission work?

In direct transmission, the pathogen passes from an infected person or animal to a new host without an intermediate object.

• Physical contact: touching, kissing or sexual contact. Examples: HIV, chlamydia, ringworm, scabies.
• Droplets: large droplets from coughing, sneezing and talking, which travel only a short distance (1 to 2 meters) before falling. Examples: influenza, colds.
• Mother to child: across the placenta, during birth or through breast milk. Examples: HIV, rubella, Zika virus.
• Bites and scratches from animals: rabies is passed in the saliva of an infected animal.

How does indirect transmission work?

In indirect transmission, the pathogen is carried by something between hosts.

• Airborne: tiny particles (aerosols) that remain in the air for longer, allowing spread over greater distances and in poorly ventilated rooms. Examples: measles, tuberculosis, and COVID-19 in some circumstances.
• Contaminated water: cholera, typhoid and dysentery are spread when drinking water is contaminated with feces.
• Contaminated food: Salmonella from undercooked eggs and poultry, E. coli from poor hygiene.
• Surfaces and objects (fomites): door handles, towels, needles. Hand-washing reduces the risk.
• Soil: tetanus bacteria enter wounds contaminated with soil.
• Vectors: living organisms that carry pathogens.

What are vectors?

A vector is an animal that transmits a pathogen between hosts. Most are blood-feeding arthropods. The pathogen may be carried on the body surface (mechanical transmission, for example flies carrying bacteria on their feet) or may develop or multiply inside the vector (biological transmission).

• Anopheles mosquito: malaria. A female mosquito picks up Plasmodium when feeding on an infected person. The parasite develops in the mosquito and is passed on to the next person bitten.
• Aedes mosquito: dengue, yellow fever, Zika, chikungunya.
• Tsetse fly: sleeping sickness.
• Fleas: plague, from rats to humans. Ticks: Lyme disease.
• Houseflies: can carry the bacteria of diarrheal diseases from feces to food.

Control of vectors, with insecticide-treated bed nets, removal of standing water where mosquitoes breed and spraying, has strongly reduced the number of malaria cases in many regions, although malaria still caused roughly 600 000 deaths in 2022, mostly of children in sub-Saharan Africa, according to WHO estimates.

Diseases and how they spread

How can the chain of infection be broken?

The chain of infection has links: pathogen, reservoir (where it lives), portal of exit, mode of transmission, portal of entry and susceptible host. Breaking any link stops the spread.

• Hygiene: hand-washing with soap, safe food preparation and clean water.
• Sanitation: sewage treatment and safe disposal of waste.
• Vector control: bed nets, insecticides, draining breeding sites.
• Barriers: condoms for sexually transmitted infections; masks in respiratory outbreaks.
• Isolation and quarantine: separating infected or exposed people.
• Vaccination: makes hosts immune. Herd immunity protects people who cannot be vaccinated if enough people are immune.
• Treatment: antibiotics or antivirals reduce the time that a patient is infectious.

Worked example: analyzing the spread of two diseases

A village has an outbreak of diarrhea after a flood. A month later, there are cases of fever in the village, which is located near a pond with stagnant water. Identify the likely routes and suggest control measures.

1. Diarrhea after a flood: floods can contaminate drinking water with sewage. The likely pathogen is a bacterium such as Vibrio cholerae, spread indirectly through water. The key is to protect water: boiling or chlorinating drinking water, providing latrines away from wells, and oral rehydration salts for patients.
2. Fever near the pond: stagnant water is a breeding site for mosquitoes. A recurring fever with chills is typical of malaria, a protozoan disease with a mosquito vector. The key is vector control: bed nets, draining or treating the pond and early treatment with antimalarial drugs.
3. Comparison: in the first case, the pathogen is a bacterium in water, so hygiene and clean water break the chain. In the second, it is a protozoan with a vector, so controlling the mosquito breaks the chain, and antibiotics would not help.

The conclusion needs to be tested: clinical tests, such as examining a blood smear for Plasmodium or growing bacteria from stool samples, confirm the diagnosis. The reasoning shows the importance of matching control methods to the route of transmission.

How was disease transmission discovered?

John Snow traced the 1854 cholera outbreak in London to a contaminated water pump in Broad Street, and had the handle removed. Ignaz Semmelweis showed in 1847 that hand-washing with chlorinated lime reduced deaths from childbed fever. Robert Koch’s work in the 1870s and 1880s linked specific microbes with specific diseases through Koch’s postulates. Ronald Ross showed in 1897 that mosquitoes transmit malaria and was awarded the Nobel Prize in Physiology or Medicine in 1902.

Which mistakes are common?

Mistake 1: Saying that vectors cause disease

The pathogen causes disease. The vector only carries it from host to host.

Mistake 2: Thinking all infectious diseases spread through the air

Many spread by water, food, contact or vectors. The control measures depend on the route.

Mistake 3: Using antibiotics for viral or protozoal diseases

Antibiotics act on bacteria. Other drug types are needed for other pathogens.

Mistake 4: Mixing up communicable and non-communicable diseases

Communicable diseases are caused by pathogens that spread. Diabetes and cancer, for example, do not spread from person to person.