Reactivation of latent human herpesviruses (HHV) — EBV, VZV, HSV-1, CMV, and the roseoloviruses HHV-6 and HHV-7 — is one of the most consistently documented physiological consequences of long-duration spaceflight. Across NASA's three-decade sampling programme, herpesvirus shedding has been documented in 53% of astronauts on short-duration shuttle missions and 61% on long-duration ISS missions, with shedding frequency, viral copy number, and duration all increasing with mission length (Mehta et al., 2017; Rooney et al., 2019). The mechanism is well-characterised: spaceflight activates the HPA and SAM axes, elevating cortisol and catecholamines and suppressing cell-mediated immunity (with declines in CD8+ Tcell function and NK-cell cytotoxicity of approximately 50% by flight day 90), thereby compromising the surveillance mechanisms that maintain latency (Crucian et al., 2018; Bigley et al., 2019). The first published case of HSV-1 dermatitis during a long-duration ISS mission (Mehta et al., 2022) demonstrates that reactivation can progress from asymptomatic shedding to clinical manifestation. The dialectical question for missions beyond low-Earth orbit — extended lunar missions and Mars transits of approximately 6–9 months one-way — is whether the risk established for ≤6-month ISS missions will scale linearly, sub-linearly, or super-linearly with mission duration, and whether available antiviral countermeasures (acyclovir, valacyclovir prophylaxis) can be deployed under exploration-class operational constraints. In this article I review the 2016–2022 literature and propose, as the original contribution, the Spaceflight Herpesvirus Reactivation Risk Index (SHERRI) — a normalised composite metric on [0,1] integrating five dimensions (baseline seroprevalence, shedding rate, viral copy number, clinicalmanifestation probability, and countermeasure availability) that returns a quantitative per-virus risk ranking for mission planning. Applied to the six principal latent herpesviruses, SHERRI returns the highest risk for EBV (≈0.62) and VZV (≈0.58), intermediate scores for CMV (≈0.45) and HSV-1 (≈0.50), and lower scores for HHV-6 and HHV-7