Fear causes stress. Chronic fear causes chronic stress. Chronic stress causes chronic cortisol. Chronic cortisol causes immunosupression.
- lack of sleep/exercise (which destroys the body's immunoregulatory capacities, leading to either a sort of acute immunosenescence like you said wrt fear, or alternatively leading to an overreactive immune system that kills via cytokine storm)
- decline in social interaction
- more time spent inside (which apart from the other correlates, very obviously leads to less sunlight). sunlight => vitamin d + nitric oxide; vitamin d is critical for respiratory pathology specifically, as well as just general health, and nitric oxide is very important as an immunoregulatory compound and as a preventer of strokes
- the general environment of fear/stress/anxiety (again, going to screw up the immunoregulatory balance of the body)