Gingivitis and periodontitis can worsen with reduced immune fitness. Various causes can reduce immune fitness in a host, as a result of which the balance between the host and the microbiome is disturbed. Among others, lifestyle factors, such as stress and smoking, can have a negative influence on immune fitness. An association has been demonstrated between stress and periodontitis and also acute necrotising ulcerative gingivitis or periodontitis. There are indications that neurons are able to secrete pro-inflammatory cytokines and chemokines that worsen chronic inflammatory reactions in the periodontium and compromise immune fitness. In vitro studies show high cortisol levels may contribute to the increased growth of P. gingivalis. Stress as a risk factor for periodontitis and the role of stress as a negative influence on the results of periodontal treatment are difficult to estimate clinically. Nevertheless, attention to and awareness of stress as an aspect of the comprehensive set of risk factors for periodontitis can diminish its negative impact on immune fitness.