2026 ICD-10-CM Diagnosis Code B00.4Herpesviral encephalitis

ICD-10-CM Codes›A00–B99›B00-B09›B00

ICD-10-CM B00.4
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

B00.4 is a billable ICD-10-CM diagnosis code for herpesviral encephalitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 974 through 976. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.

Code Identity

ICD-10-CM Code
B00.4
Billable Status
Yes — Valid for Submission
Code Describes
Herpesviral encephalitis
Short Description
Herpesviral encephalitis
Same as the full description in the CMS dataset.
Parent Code
B00
Herpesviral [herpes simplex] infections

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionB00-B09Viral infections characterized by skin and mucous membrane lesions
CategoryB00Herpesviral [herpes simplex] infections
This CodeB00.4Herpesviral encephalitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute encephalitis
  • Acute necrotizing encephalitis
  • Acute viral encephalitis
  • Dementia associated with viral encephalitis
  • Dementia due to herpes encephalitis
  • Encephalitis caused by Herpesvirus
  • Encephalitis caused by human herpes simplex virus
  • Herpes simiae encephalitis
  • Herpes simplex meningitis
  • Herpes simplex type 1 encephalitis
  • Herpes simplex type 1 infection
  • Herpes simplex type 2 encephalitis
  • Herpes simplex type 2 infection
  • Herpetic acute necrotizing encephalitis
  • Meningoencephalitis caused by human herpes simplex virus
  • Meningoencephalitis caused by virus
  • Simian B disease
  • Simian B encephalomyelitis

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Herpesviral meningoencephalitis
  • Simian B disease

Type 1 Excludes

  • herpesviral encephalitis due to herpesvirus 6 and 7 B10.01 B10.09
  • non-simplex herpesviral encephalitis B10.0

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Disease, diseased
      • simian B
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • herpesviral
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • in (due to)
        • herpes (simplex) virus
    • Herpes, herpesvirus, herpetic
      • encephalitis
    • Meningoencephalitis
      • herpesviral, herpetic
    • Meningoencephalitis
      • in (due to)
        • herpes

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR INF008
Viral infection
Default principal diagnosis: inpatient No · outpatient No

Clinical InformationClinical

  • Acute Necrotizing Encephalitis

    a rare infectious encephalitis primarily affecting children that is characterized by rapid neurological deterioration and most often follows influenza infection.

Patient EducationClinical

Encephalitis

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

The full article covers:

  • What is encephalitis?
  • What causes encephalitis?
  • Who is more likely to get encephalitis?
  • What are the symptoms of encephalitis?
  • How is encephalitis diagnosed?
  • What are the treatments for encephalitis?
  • Can encephalitis be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert B00.4 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
054.3 Herpetic encephalitis
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About B00.4Overview

Is B00.4 (Herpesviral [herpes simplex] infections) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report herpesviral encephalitis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does B00.4 group to?

When herpesviral encephalitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 974, 975, 976, with relative weights from 0.8945 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of B00.4?

Under the General Equivalence Mappings, herpesviral encephalitis converts to ICD-9-CM 054.3 (herpetic encephalitis). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.