Definition
A latent infection is a state in which a pathogen persists within a host without active replication or clinical symptoms. The organism remains viable and capable of reactivation under certain conditions.
Explanation
In latent infections, the host immune response contains but does not eradicate the infectious agent. The pathogen adopts a dormant or low‑replicative state, often within specific tissues or cells, allowing it to evade immune surveillance and antimicrobial therapy. Viral latency is exemplified by herpesviruses, which maintain their genomes as episomal or integrated DNA and express only latency‑associated transcripts; reactivation triggers viral replication and disease recurrence. Bacterial latency occurs when organisms such as Mycobacterium tuberculosis reside within granulomas, where hypoxia and nutrient limitation lead to metabolic downregulation. Latent infections differ from chronic infections in that there is minimal or no ongoing replication and the host is asymptomatic. Diagnosis may rely on immunologic tests (e.g., tuberculin skin test, interferon‑γ release assays for latent tuberculosis), serologic markers (HSV antibodies) or detection of viral DNA in tissues. Factors that can reactivate latent infections include immunosuppression, stress, aging and co‑infection. Understanding the biology of latent reservoirs is crucial for disease control, as reactivation can lead to secondary outbreaks and transmission.
Notable examples and considerations
• Latent tuberculosis infection affects about a quarter of the world’s population; only a small fraction progress to active TB, but reactivation risk increases with HIV infection and immunosuppressive therapy.
• Herpes simplex viruses establish latency in sensory neurons; stimuli such as fever or UV exposure can trigger recurrent lesions.
• Varicella zoster virus remains latent in dorsal root ganglia after chickenpox; reactivation results in shingles and postherpetic neuralgia.
• Cytomegalovirus and Epstein–Barr virus persist in hematopoietic cells and may reactivate in transplant recipients, causing systemic disease.
• Antimicrobial therapy that targets actively replicating organisms may not eliminate latent reservoirs, highlighting the need for preventive treatment in some cases (e.g., isoniazid for latent TB).
Latent infections represent a balance between host defenses and pathogen persistence. Monitoring and managing latent infections are essential to prevent reactivation and transmission.
Related Terms: latency, persistent infection, reactivation, granuloma, carrier state