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Education Materials > Publications > Self-Study Modules on TB > Module 5 > Infectiousness

Self-Study Modules on Tuberculosis

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Module 5: Infectiousness and Infection Control


What Factors Affect the Infectiousness of a TB Patient?

The infectiousness of a TB patient is directly related to the number of tubercle bacilli that he or she expels into the air. Patients who expel many tubercle bacilli are more infectious than patients who expel few or no bacilli. The number of tubercle bacilli expelled by a TB patient depends on the following factors.

What is the site of the disease?

Usually, only people with pulmonary or laryngeal TB (TB of the larynx) are infectious. This is because these people may be coughing and expelling tubercle bacilli into the air. People with extrapulmonary TB only (no pulmonary TB) generally are not infectious. This is because tubercle bacilli usually cannot be expelled into the air from an extrapulmonary site.

Does the chest x-ray show that the patient has a cavity in the lung?

Because there are many tubercle bacilli in a cavity, patients who have a cavity in the lung may be expelling tubercle bacilli if they are coughing.

Is the patient coughing? If so, how often and how forcefully?

Patients expel more tubercle bacilli if they have a cough that produces a lot of sputum. Also, they may expel tubercle bacilli if they are undergoing medical procedures that cause them to cough (cough-inducing procedures).

Does the patient cover his or her mouth when coughing?

Patients who do not cover their mouth when they cough are more likely to expel tubercle bacilli.

Are there acid-fast bacilli on the sputum smear?

The presence of acid-fast bacilli on a sputum smear indicates that the patient may be expelling tubercle bacilli.

Is the patient receiving adequate treatment?

Patients who have NOT been receiving adequate treatment are much more likely to be infectious than patients who have been receiving adequate treatment for 2 to 3 weeks or longer. Patients who have been receiving adequate treatment usually respond to treatment; in other words, their symptoms improve and eventually go away.

Also, patients who have drug-resistant TB are more likely to be infectious than patients who have drug-susceptible TB. This is because patients with drug-resistant TB may not respond to the initial drug regimen, and they may remain infectious until they receive proper drugs.

These factors are summarized in Table 5.1.

Table 5.1
Infectiousness of People Known to Have or 
Suspected of Having TB Disease*
Factors Associated with Infectiousness Factors Associated with Noninfectiousness
TB of the lungs or larynx Most extrapulmonary TB
Cavity in the lung No cavity in the lung
Cough or cough-inducing procedures No cough or cough-inducing procedures
Patient not covering mouth when coughing Patient covering mouth when coughing
Acid-fast bacilli on sputum smear No acid-fast bacilli on sputum smear
Not receiving adequate treatment Receiving adequate treatment for 2-3 weeks
* Infectiousness depends on a variety of factors. Clinicians should consider all of these factors when determining whether a TB patient should be considered infectious.

Young children with pulmonary or laryngeal TB disease are much less likely than adults to be infectious. This is because children generally do not produce sputum when they cough. However, it is possible for children to transmit TB to others.

Infectiousness appears to decline very rapidly after adequate treatment is started, but how quickly it declines varies from patient to patient. Some patients may stop being infectious on the day they begin treatment. Others may remain infectious for weeks or even months. Patients with drug-resistant TB may not respond to the initial drug regimen, and they may remain infectious until they receive proper drugs.

Patients can be considered noninfectious when they meet all of the following criteria:

  • They have been receiving adequate treatment for 2 to 3 weeks
  • Their symptoms have improved (for example, coughing less and no longer have a fever)
  • They have THREE consecutive negative sputum smears from sputum collected on different days

Study Questions 5.1-5.3

5.1. Why does the site of disease affect the infectiousness of a TB patient?

5.2. List five other factors that affect the infectiousness of a TB patient.

5.3. When can a TB patient be considered noninfectious? List all three criteria.



Case Study 5.1

For each of the following situations, decide whether the patient should be considered infectious or noninfectious, and explain why.

  • Mr. Lopez started TB treatment 7 days ago. He still has a cough. Two weeks ago, he had a sputum smear that was positive; since then no sputum specimens have been tested.
  • Ms. Nguyen, a patient with pulmonary TB, has been receiving TB treatment for 6 weeks, and she no longer has symptoms of TB. She has had three sputum smears done. The first one was positive, but the last two were negative.
  • Mr. Martin started treatment for pulmonary TB in April. His symptoms went away and his sputum smears became negative in May. He missed his clinic appointment in June. When he returned to the TB clinic at the beginning of August, he was coughing.



Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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