Breakthrough infection mice exhibit lung eosinophil infiltration that peaks at 7–10 days post-challenge, enriched for the Siglec-Fhi subset, but in the absence of overt pro-inflammatory cytokine/chemokine signals, high viral titers, severe lung lesions, goblet cell hyperplasia, allergic levels of total IgE, or enhanced morbidity. The gene discussed is IGHE; the disease is infection.