In general, the expression of CTLA-4 in the PCV2 alone group and the PCV2–PRRSV infected group increased with the time since infection, while the expression of CTLA-4 in the PRRSV alone group, the PRRSV–PCV2 group and the PCV2 + PRRSV group reached a peak at 36 hpi. The gene discussed is CTLA4; the disease is infection.