Can stress really make you sick?

Absolutely. From APA:

The results revealed a clear relationship between the amount of life stress reported and the percentage of people who caught the cold. As stress increased, so did the incidence of the cold, and this relationship was strongly linear, meaning that for each increment of additional stress, there is an additional increment of susceptibility to the cold.

And:

In a follow-up study, the result was replicated with an improved measure of cold infection. Cohen also obtained more information about the types of stress people were experiencing, and discovered that chronic stressors produce much more susceptibility than acute, short term stressors. Interpersonal conflicts and work-related stress due to over or unemployment also led to more infections than other forms of enduring stressors.

Psychological factors, such as stress, are a good predictor of who will get a cold in a controlled environment. Other psychosocial factors are also important. For example, individuals with few high-contact social roles had more colds, and the less diverse their social network, the more colds they had as well.

Cohen and his colleagues questioned how an individualâs personality might affect cold susceptibility. It wasnât long before they found that introverts get sick more often, although this didnât explain the social role effect fully.

Health practices also explained some of the differences in susceptibility, though some may come as a surprise: alcohol consumption had an effect, but it was the nondrinkers who had twice the risk factor for the cold compared to drinkers. Lack of exercise also doubled the risk factor, and smoking increased it threefold. Poor sleep and vitamin C intake are also related. Yet, none of these differences explained the effect of stress, meaning that stress has its impact via something other than health-related behaviors.