From exposure to disease
A pathogen is a microbe able to cause disease. Most microbes people meet are harmless or helpful. Pathogenesis is the process by which disease develops, and virulence is the degree of harm a pathogen can cause. One way to compare virulence is the ID50, the dose that infects half of exposed hosts: a lower dose means higher virulence. A pathogen usually needs to reach a portal of entry, adhere, invade or evade defences, and then damage the host.
Toxins
Exotoxins are proteins secreted by bacteria, often very potent and specific. Endotoxin is the lipid A part of LPS in the outer membrane of Gram-negative bacteria, and is released when those cells die; it can provoke fever and strong inflammation.
Stages of an infectious disease
Typically: incubation (no symptoms), prodromal (vague early signs), illness (peak), decline, and convalescence. A person may transmit before symptoms appear.
Chain of transmission
A reservoir (people, animals, soil) holds the pathogen. It passes by direct contact, by droplets, through the air, by food or water, by a vector such as a mosquito or tick, or via a fomite, an inanimate object like a doorknob. Diseases spread from animals to people are zoonotic. Healthcare-associated infections are acquired in care settings.
Epidemiology vocabulary
- Incidence: new cases in a period, per population size.
- Prevalence: all existing cases, old and new, at a point in time.
- Sporadic: occasional cases. Endemic: steadily present in a region. Epidemic: cases above what is expected. Pandemic: an epidemic across several continents or worldwide.
- Herd immunity: enough immune people that spread slows.
- R0, the basic reproduction number, is the average number of new cases from one case in a fully susceptible population. Spread grows when it is above 1.
- Koch's postulates link one microbe to one disease: found in every case, grown in pure culture, causes the disease in a healthy host, and re-isolated.
Worked example
A village of 8,000 people has 40 people newly ill with a respiratory infection in March, and 120 people ill at the end of March from cases that began in February or March.
- March incidence is 40 divided by 8,000, which is 0.005, or 5 per 1,000 people.
- Point prevalence is 120 divided by 8,000, which is 0.015, or 1.5%.
Suppose R0 is 3 and no one is immune. One case yields 3 new cases, and these yield 9, then 27. Notice incidence counts only new cases, while prevalence includes long-lasting ones, so a chronic disease can have low incidence and high prevalence.