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# Spectrum of infection: inapparent, clinical and carriers

- Concept

Most infections go unseen: clinical cases are the tip of the iceberg, and carriers spread infection without knowing it.

Updated Nov 13, 2026

## Key points

- Infectivity = infected / exposed susceptibles.
- Pathogenicity = clinical cases / infected.
- Virulence = severe or fatal cases / clinical cases.
- Carriers: asymptomatic (healthy), incubatory, convalescent and chronic (hepatitis B, typhoid fever).
- Incubation period: from infection to symptoms. Latent period (in infections): from infection to the start of infectiousness.
- If the latent period is shorter than the incubation period, transmission happens before symptoms.

On this page

- The basics Key points
- What it is
- Worked example
- Types of carrier
- Markers
- In practice
- Common pitfalls
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

The same infection produces, across a population, everything from inapparent infections to severe or fatal cases: the spectrum, or iceberg, of infection. Three proportions describe it: infectivity (exposed people who become infected), pathogenicity (infected people who become ill) and virulence (ill people with severe or fatal disease). Carriers, with inapparent infection but able to transmit, and the relation between the incubation and latent periods explain why many outbreaks cannot be controlled by isolating the sick alone.

## Worked example

- Outbreak in a school: 400 pupils exposed and susceptible; 300 have new antibodies (infected).
- Infectivity = 300/400 = 75%.
- 120 of the 300 infected had symptoms: pathogenicity = 120/300 = 40%.
- 12 of the 120 cases were severe or admitted to hospital: virulence = 12/120 = 10%.
- Inapparent infections: 300 − 120 = 180, 60% of those infected.
- Reading: surveillance that counts only symptoms sees 120 of 300 infections; 180 could transmit unknowingly.

## Types of carrier

| Type | When they transmit | Example |
| --- | --- | --- |
| Asymptomatic (healthy) | Without ever becoming ill | Pharyngeal carrier of meningococcus |
| Incubatory | Before symptoms appear | Hepatitis A, shed in stool before symptoms |
| Convalescent | After recovering | Some bacterial diarrhoeas, such as salmonellosis |
| Chronic | For months or years | Hepatitis B, typhoid fever |

## Markers

- Infectivity
- Pathogenicity
- Virulence
- Carrier

## In practice

- Serology or testing of contacts reveals the hidden part of the iceberg and changes the true size of the outbreak.
- With presymptomatic or asymptomatic transmission, isolating only the sick is not enough: contacts are traced and vaccination is used.
- A case fatality calculated only from severe hospital cases overestimates how dangerous the infection is.

## Common pitfalls

- Confusing the latent period (start of infectiousness) with the incubation period (start of symptoms).
- Using the infected rather than the clinical cases as the denominator of virulence.
- Assuming that an infection without symptoms does not transmit.

## Memory aid

Infectivity opens the door, pathogenicity turns on the light and virulence breaks the furniture: each one counted among those who passed the step before.

## Test yourself

Think of the answer, then tap the question to see it.

## 300 infected out of 400 exposed and 120 with symptoms. Infectivity and pathogenicity?

Infectivity 300/400 = 75%; pathogenicity 120/300 = 40%.

## What is the denominator of virulence?

Clinical cases: virulence = severe or fatal cases / clinical cases.

## What is a convalescent carrier?

Someone who has recovered from the illness but still sheds the agent and can transmit it.

## What does a latent period shorter than the incubation period imply?

That people are infectious before they have symptoms, so isolating symptomatic people does not stop all transmission.

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## Elsewhere in DurgiPedia

- [Herd immunity and outbreak control](https://durgical.com/en/immunology/herd-immunity-and-outbreak-control/)

## Connections

## Related 6

- [Natural history of disease](https://durgical.com/en/epidemiology/natural-history-of-disease/)
- [Epidemiologic triad and chain of infection](https://durgical.com/en/epidemiology/epidemiologic-triad-and-chain-of-infection/)
- [Attack rate](https://durgical.com/en/epidemiology/attack-rate/)
- [Case fatality rate](https://durgical.com/en/epidemiology/case-fatality-rate/)
- [Basic reproduction number (R0) and herd immunity](https://durgical.com/en/epidemiology/basic-reproduction-number-and-herd-immunity/)
- [Outbreak investigation](https://durgical.com/en/epidemiology/outbreak-investigation/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology | Lesson 1 - Section 9](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson1/section9.html) (Centers for Disease Control and Prevention, 2012); [Principles of Epidemiology | Lesson 1 - Section 10](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson1/section10.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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