ISG15 and infection: Our main findings were: 1) the host immune response was initiated only in fetuses with detectable levels of PRRSV; 2) upon infection of fetal thymus, a set of core responsive IFN-inducible genes (CXCL10, IFIH1, IFIT1, IFIT3, ISG15, and MX1) were strongly upregulated in both tissues.