This phenomenon is called hypoimmunity stage after exercise. After strenuous exercise (maybe no exercise for a long time), the immunity will decrease within 1-8 hours. This will happen after anaerobic resistance training and aerobic exercise. You can do exercise step by step, gradually increase the intensity, or you can add enough protein after exercise. Glucose is an economical choice for rapidly supplementing muscle glycogen (for thin people). Glutamine can also be used.