Final answer:
The presence of acid-fast bacilli is the surrogate marker for infectivity for tuberculosis. Patients with three negative smears are considered non-infectious for tuberculosis.
Step-by-step explanation:
The surrogate marker for infectivity for tuberculosis is the presence of acid-fast bacilli. Acid-fast bacteria can be detected through microscopy, specifically using the Ziehl-Neelsen staining technique. If a patient has three negative smears, they are considered non-infectious for tuberculosis.