A substantial increase in lung macrophage prevalence has been demonstrated in bronchoalveolar lavage fluid from nine COVID-19 patients (Liao et al. 2020), and the proportion of those which were monocyte-derived Fcn1+ macrophages—rather than tissue-resident alveolar macrophages—progressively increased from healthy controls (N = 3) to patients with mild COVID-19 (N = 3) and to patients with severe disease (N = 6). The gene discussed is FCN1; the disease is COVID-19.