Comorbidity and Risk Factors for Infection

Coexisting chronic conditions and other host characteristics raise the risk of acquiring infection and of suffering severe disease. Diabetes, chronic lung, heart, liver, and kidney disease, obesity, and similar conditions impair defences or create vulnerable tissue, while behavioural and environmental factors add further risk. This topic organises comorbidity and other host risk factors as determinants of infection susceptibility and severity at the population level.

Definition

Comorbidity and risk factors for infection are coexisting diseases (such as diabetes or chronic organ disease) and other host or environmental characteristics that increase the probability of acquiring an infection or of progressing to severe disease, by impairing immune or barrier defences or by increasing exposure.

Scope

The topic covers comorbidity and host-level risk factors as determinants of who becomes infected and who develops severe disease, treated epidemiologically rather than as clinical management of any specific condition. It addresses how chronic diseases impair host defence and how risk factors combine, and it connects to age and immunocompromise, which frequently coincide with comorbidity. It does not provide diagnostic criteria or treatment for the underlying conditions themselves.

Core questions

• Which chronic conditions raise the risk of infection and of severe disease, and why?
• How does comorbidity impair host defence at the mechanistic level?
• How do multiple risk factors combine to shape an individual's or group's infection risk?
• How does comorbidity interact with age and immunocompromise as determinants of variation?

Key concepts

• Comorbidity
• Chronic disease and impaired host defence
• Risk factors for infection
• Multimorbidity
• Susceptibility and severity
• Confounding by comorbidity

Mechanisms

Chronic conditions impair host defence through varied routes. Geerlings and Hoepelman describe how diabetes mellitus is associated with immune dysfunction, including impaired neutrophil and other immune-cell function, which helps explain the heightened infection risk in people with diabetes. More broadly, chronic lung, heart, liver, and kidney disease can damage barrier tissues, reduce clearance, or blunt immune responses, while conditions and treatments that suppress immunity overlap with immunocompromise. Because such conditions cluster with age and with one another, their effects on infection risk often accumulate, and in epidemiological analysis comorbidity is both a genuine risk factor and a frequent confounder of other exposure-outcome associations.

Clinical relevance

Comorbidity is widely used to characterise populations at elevated risk of infection and severe outcomes, and to interpret why burden concentrates in people with chronic disease. This topic is a reference for understanding comorbidity as a determinant of infection variation; it summarises population-level associations and mechanisms and does not provide individual diagnostic or treatment recommendations.

Epidemiology

Underlying conditions are consistently associated with higher infection incidence and severity. People with diabetes have an increased risk of several infections, linked to the immune dysfunction reviewed by Geerlings and Hoepelman. During the coronavirus disease 2019 pandemic, severity and death were concentrated in people with comorbidities as well as older age, as reflected in the severity estimates of Verity and colleagues, illustrating how coexisting conditions shape the population distribution of severe infectious disease.

Key figures

• Suzanne Geerlings
• Andy Hoepelman

Seminal works

• geerlings-hoepelman-1999
• verity-2020

Frequently asked questions

Why does having a chronic disease raise the risk of infection?

Many chronic conditions impair host defences, for example by damaging barrier tissues, reducing clearance of microbes, or blunting immune-cell function. Diabetes, for instance, is associated with immune dysfunction that increases susceptibility to several infections.

Is comorbidity a risk factor or a confounder in infection studies?

It can be both. Comorbidity genuinely raises infection risk and severity, and because it clusters with age and other exposures it also frequently confounds associations between other risk factors and infection outcomes.