Step 1 topics → Microbiology → Herpesvirus Family (HSV-1/2, VZV, EBV, CMV, HHV-6/7/8)

Herpesvirus Family (HSV-1/2, VZV, EBV, CMV, HHV-6/7/8)

USMLE Step 1 trap: Rigidly assigns HSV-1 to oral and HSV-2 to genital disease, ignoring significant overlap. While HSV-1 classically causes orolabial herpes and HSV-2 causes genital herpes, either type can infect either site; HSV-1 is now a leading cause of primary genital herpes in many populations.

Herpesviruses are one of the highest-yield virus families on USMLE Step 1, and the EBV peripheral smear question is the single most consistently answered incorrectly: EBV infects B cells via the CD21 receptor, but the atypical lymphocytes (Downey cells) you see on the peripheral smear are reactive CD8+ cytotoxic T cells — not EBV-infected B cells. They look atypical because they're activated, not infected. Getting this wrong on a smear question is a one-point loss that's easily prevented. The family as a whole is tested from pure recall (which virus causes roseola) to clinical application (HIV patient with violaceous lesions = HHV-8, not HIV itself) to passage-based congenital infection distinctions (CMV periventricular calcifications vs. toxo diffuse calcifications).

The tricky part isn't learning the individual viruses — it's keeping them straight under pressure. EBV and CMV both cause mononucleosis-like syndromes. Congenital CMV and congenital toxoplasmosis both cause intracranial calcifications. HSV-1 and HSV-2 overlap more than most students expect. HHV-8 gets incorrectly credited to HIV. These are not random mistakes — they reflect predictable gaps in how students build their mental models, and USMLE Step 1 is very good at engineering questions that target exactly those gaps.

To master this topic, don't just memorize each virus in isolation. Build comparison tables in your head: HSV-1 vs HSV-2 (where they infect, where they latent, what encephalitis pattern), CMV vs toxo congenital (periventricular vs diffuse calcifications), EBV-infected cells vs the cells you actually see on smear. That comparative thinking is what separates a 75th percentile answer from a 95th percentile answer on these questions.

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Common misconceptions

Common mistake
Wrong: HSV-1 exclusively causes oral lesions and HSV-2 exclusively causes genital lesions.
Right: While HSV-1 classically causes orolabial herpes and HSV-2 causes genital herpes, either type can infect either site; HSV-1 is now a leading cause of primary genital herpes in many populations.
The classic teaching — HSV-1 oral, HSV-2 genital — reflects historical epidemiology, not a biological rule. Either serotype can infect any mucosal surface. In current data, HSV-1 causes a substantial proportion of primary genital herpes, particularly in younger populations, likely due to orogenital contact. When a question specifies HSV-1 genital infection, that's valid; don't dismiss it. The more reliable distinguishing features are latency site (trigeminal ganglion for HSV-1, sacral dorsal root ganglia for HSV-2) and encephalitis pattern (HSV-1 causes temporal lobe encephalitis).
Common mistake
Wrong: The atypical lymphocytes seen in EBV mononucleosis are infected B cells.
Right: EBV infects B cells, but the atypical lymphocytes (Downey cells) seen on peripheral smear are reactive CD8+ T cells responding to EBV-infected B cells.
This is one of the most commonly tested EBV distinctions. EBV selectively infects B cells via the CD21 receptor — that part is correct. But the atypical lymphocytes you see on the peripheral smear (Downey cells) are reactive CD8+ cytotoxic T cells mounting an immune response against those infected B cells. They're large, with irregular nuclei and abundant cytoplasm — they look atypical because they're activated, not because they're infected. If a question shows a smear with atypical lymphocytes and asks what those cells are, the answer is reactive T cells, not EBV-infected B cells.
Common mistake
Wrong: Congenital CMV causes diffuse intracranial calcifications like congenital toxoplasmosis.
Right: Congenital CMV causes periventricular calcifications, sensorineural hearing loss, and microcephaly, while congenital toxoplasmosis causes diffuse (non-periventricular) calcifications and chorioretinitis.
Both cause intracranial calcifications in neonates, but the distribution is the key differentiator: CMV causes periventricular calcifications (around the ventricles), while toxoplasmosis causes diffuse calcifications scattered throughout the brain parenchyma. CMV's other hallmarks are sensorineural hearing loss (most common cause of non-hereditary SNHL in neonates) and microcephaly. Toxoplasmosis features chorioretinitis and hydrocephalus more prominently. When you see 'periventricular' in a stem, go to CMV. When you see 'diffuse' or 'chorioretinitis' in the same breath, go to toxo.
Common mistake
Wrong: Herpes zoster (shingles) can cross the midline like a typical skin infection.
Right: Herpes zoster reactivation follows a single dermatome and characteristically does not cross the midline, reflecting reactivation in a single dorsal root or cranial nerve ganglion.
Herpes zoster reactivates from a single dorsal root ganglion or cranial nerve ganglion — that's why it's strictly unilateral and dermatomal. The virus reactivates, travels down sensory nerve fibers, and causes vesicular lesions confined to that dermatome. It physically cannot cross the midline because it follows neuroanatomy, not skin anatomy. If a rash crosses the midline, that's not zoster — think about other diagnoses. This dermatomal pattern is also how you distinguish zoster from primary VZV (chickenpox), which is diffuse.
Common mistake
Wrong: Kaposi sarcoma is caused directly by HIV.
Right: Kaposi sarcoma is caused by HHV-8 (KSHV); HIV-induced immunosuppression allows HHV-8 reactivation and tumor development, but HHV-8 is the direct oncogenic agent.
HIV causes immunosuppression, which allows HHV-8 (Kaposi sarcoma-associated herpesvirus, KSHV) to reactivate — but HHV-8 is the actual oncogenic driver. HIV itself does not directly cause Kaposi sarcoma. HHV-8 encodes viral homologs of cytokines and cell cycle regulators that drive spindle cell proliferation and angiogenesis, producing the characteristic violaceous lesions. This distinction matters on USMLE Step 1 when a question asks about the causative agent or mechanism — the answer is HHV-8, not HIV.
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What the exam tests

  1. HSV-1 vs HSV-2: Know the classic vs actual distribution of syndromes (orolabial, genital, encephalitis), which type latency lives in the trigeminal vs sacral ganglia, how to diagnose (Tzanck smear, PCR), and what treatment is used — including when to use IV acyclovir for HSV encephalitis.
  2. VZV primary vs reactivation: Distinguish chickenpox (diffuse vesicular rash) from shingles (dermatomal, unilateral, does not cross midline), identify complications like Ramsay Hunt syndrome and post-herpetic neuralgia, and know the two vaccine types (live-attenuated Varivax for children, recombinant Shingrix for adults ≥50).
  3. EBV mononucleosis: Know the classic triad (fever, pharyngitis, lymphadenopathy), what cells are actually infected (B cells via CD21), what cells appear on peripheral smear (reactive CD8+ T cells — Downey cells), the heterophile antibody test, risks from amoxicillin use (maculopapular rash), and serious sequelae (splenic rupture, EBV-associated malignancies including Burkitt lymphoma and nasopharyngeal carcinoma).
  4. CMV disease spectrum: Distinguish congenital CMV (periventricular calcifications, sensorineural hearing loss, microcephaly, blueberry muffin rash) from adult mononucleosis (heterophile-negative) from immunocompromised disease (retinitis, esophagitis, colitis, pneumonitis — with owl-eye inclusions on biopsy). Know that ganciclovir/valganciclovir is the treatment and that CMV retinitis is an AIDS-defining illness.
  5. HHV-6/7/8 associated diseases: Know HHV-6 causes roseola infantum (exanthem subitum) — high fever then rash as fever breaks in young children. Know HHV-8 causes Kaposi sarcoma (violaceous lesions in HIV patients) and primary effusion lymphoma, and that HHV-8 is the direct oncogenic agent — not HIV itself.

Can you avoid these mistakes?

A 19-year-old college student presents with 5 days of fever, severe sore throat, and posterior cervical lymphadenopathy. Monospot is positive. A peripheral blood smear shows abundant atypical lymphocytes. What cell type are these atypical lymphocytes, and what cell type does the causative virus actually infect?
A 2-week-old neonate has microcephaly, periventricular calcifications on head CT, and fails a newborn hearing screen bilaterally. What is the most likely diagnosis, and how does the imaging pattern differ from the other TORCH infection that also causes intracranial calcifications?
A 35-year-old man with HIV (CD4 count 50 cells/µL) develops painful eye floaters and a unilateral scotoma. Fundoscopy shows hemorrhages and white fluffy retinal lesions. What is the diagnosis, what pathogen is responsible, and what would a biopsy of infected tissue show histologically?
A 28-year-old woman presents with her first episode of painful genital vesicles. HSV PCR comes back positive for HSV-1. A classmate insists this must be wrong because HSV-1 only causes cold sores. How would you explain why the classmate is incorrect, and where does HSV-1 establish latency in this patient?

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