Self-Study Modules on Tuberculosis
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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
Does the chest x-ray show that the patient has a cavity in the
Because there are many tubercle bacilli in a cavity, patients who
have a cavity in the lung may be expelling tubercle bacilli if they
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
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
These factors are summarized in Table 5.1.
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
- 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 - http://www.cdc.gov/tb
Please send comments/suggestions/requests
CDC/Division of Tuberculosis Elimination
Communications, Education, and Behavioral Studies Branch
1600 Clifton Rd., NE - Mailstop E-10, Atlanta, GA 30333