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Microbiology · Human herpesviruses

Herpes simplex virus 1 !

  • Virus
  • Enveloped
  • Exam priority: high

The virus behind cold sores; it sleeps in the nerves of the face and rarely causes severe encephalitis.

Updated 2026-11-01

Also called: HSV-1, cold sores, fever blisters

At a glance

Shape
Icosahedral with tegument and envelope
Genome
Linear double-stranded DNA
Envelope
Enveloped
Family
Herpesviridae (alphaherpesvirus)
Size
About 150 to 200 nm

Morphology

  • Icosahedral capsid surrounded by tegument and a lipid envelope
  • Forms multinucleated giant cells with intranuclear (Cowdry A) inclusions

Transmission

  • Saliva and contact with sores or the skin around the mouth
  • Oral-genital contact: a growing cause of genital herpes
  • Can spread without visible lesions (asymptomatic shedding)

Reservoir: Humans

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Latency in neurons of the trigeminal ganglion, with no virions made
  • Reactivation by fever, sunlight, stress, menstruation, injury or surgery
  • Envelope glycoproteins that allow cell-to-cell fusion
  • The viral thymidine kinase activates antivirals; its mutation causes resistance

Diseases it causes

Clinical picture

  • Grouped vesicles on a red base that ulcerate and crust
  • Primary infection in children: gingivostomatitis with fever and painful eating
  • Encephalitis: fever, confusion, seizures and temporal focal signs; a common cause of sporadic viral encephalitis

Diagnosis

  • PCR of lesion fluid (the most sensitive test)
  • HSV PCR of cerebrospinal fluid for encephalitis
  • MRI showing temporal lobe involvement in encephalitis
  • The Tzanck smear (multinucleated giant cells) is no longer recommended: it cannot tell HSV from VZV

Treatment

Nucleoside analogues: acyclovir, valacyclovir and famciclovir; in encephalitis, intravenous acyclovir is started as soon as it is suspected. For acyclovir-resistant strains (thymidine kinase mutation), foscarnet.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Avoid direct contact with active sores
  • Suppressive therapy for frequent recurrences
  • Standard precautions; contact precautions for disseminated disease or severe primary infection

Test yourself

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

Which brain lobe does HSV-1 encephalitis typically affect?

The temporal lobe, often with asymmetric, hemorrhagic involvement.

In which ganglion does HSV-1 stay latent?

The trigeminal ganglion; on reactivation it travels down the nerve and causes cold sores.

Which test confirms HSV encephalitis?

HSV PCR of cerebrospinal fluid.

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← Back to the group: Human herpesviruses

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Herpes simplex virus (WHO, 2024); Herpes - STI Treatment Guidelines (CDC, 2021); Appendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions (CDC, 2024).

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