Pathogens are organisms or particles that can cause disease when they enter a host and disrupt normal body functions. They include bacteria, viruses, fungi, and protists, each with different structures and ways of reproducing. Understanding how pathogens spread helps people predict outbreaks, protect communities, and choose effective prevention methods.
Disease transmission is not random because it follows patterns based on biology, behavior, and environment.
The chain of infection describes the steps a pathogen needs to move from one host to another. These steps are infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. Breaking any link in the chain can stop disease spread, such as washing hands to block transmission or vaccination to reduce susceptibility.
Public health strategies combine personal hygiene, sanitation, safe food practices, vector control, isolation, and immunization.
Key Facts
- The 6 links in the chain of infection are infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host.
- Bacteria are living single-celled organisms, while viruses are nonliving particles that must use host cells to reproduce.
- Direct transmission occurs through close contact, while indirect transmission can occur through contaminated objects, food, water, air, or vectors.
- Basic reproduction number R0 is the average number of new infections caused by one infected person in a fully susceptible population.
- If R0 > 1, an infection can spread through a population; if R0 < 1, the outbreak tends to decline.
- Vaccination reduces the number of susceptible hosts and can lower the effective reproduction number: Re = R0 x S, where S is the fraction of the population still susceptible.
Vocabulary
- Pathogen
- A pathogen is a bacterium, virus, fungus, protist, or other agent that can cause disease in a host.
- Reservoir
- A reservoir is a place where a pathogen normally lives, grows, or survives, such as a person, animal, soil, or water.
- Transmission
- Transmission is the movement of a pathogen from a source to a new host.
- Vector
- A vector is a living organism, such as a mosquito or tick, that carries a pathogen from one host to another.
- Susceptible Host
- A susceptible host is an individual who can become infected because they lack enough immunity or protection against a pathogen.
Common Mistakes to Avoid
- Calling all pathogens germs without distinguishing types is wrong because bacteria, viruses, fungi, and protists respond to different treatments and prevention methods.
- Assuming antibiotics cure viral infections is wrong because antibiotics target bacterial structures or processes that viruses do not have.
- Forgetting the portal of entry is wrong because a pathogen must enter the body through a specific route, such as the mouth, nose, eyes, skin breaks, or reproductive tract.
- Thinking prevention requires killing every pathogen is wrong because disease spread can be stopped by breaking any link in the chain of infection.
Practice Questions
- 1 A disease has R0 = 3 in a fully susceptible population. If 1 infected person enters the population, how many new infections are expected in the next generation of spread?
- 2 A class has 30 students. If 24 students are immune to a pathogen, what fraction of the class is still susceptible, and what is Re if R0 = 2.5?
- 3 Explain how handwashing, mosquito control, and vaccination each break a different link in the chain of infection.