The term “susceptibility” refers to the likelihood an individual will contract a specific disease or experience severe negative outcomes after pathogen exposure. Public health agencies classify certain groups as “highly susceptible” to prioritize protective measures, such as vaccines, specialized care, and targeted public health messaging. This classification allows for focused interventions to protect those most likely to face hospitalization, severe illness, or death from common infections.
Defining High Susceptibility in Public Health
Public health organizations, including the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), define a highly susceptible population based on measurable metrics indicating a significantly increased risk compared to the general population. This classification is used for risk stratification, guiding policy decisions on who should be prioritized for preventive and therapeutic measures. A group is designated as highly susceptible if they demonstrate higher rates of infection, increased probability of hospitalization, or greater mortality following exposure to communicable diseases. The designation is based on the body’s diminished ability to mount an effective immune response or withstand the physiological stress of the illness.
Medically Fragile and Immunocompromised Populations
Highly susceptible individuals include those who are medically fragile or have a compromised immune system, which impairs the body’s defense mechanisms against pathogens. Chronic health conditions increase susceptibility because they weaken the body’s overall resilience or directly interfere with immune function.
For instance, individuals with Type 1 or Type 2 diabetes may have difficulty fighting off infections due to high blood glucose levels, which can impair white blood cell function. Conditions affecting the cardiovascular system, such as heart failure, also place individuals at higher risk for severe outcomes from respiratory viruses. Chronic lung diseases like Chronic Obstructive Pulmonary Disease (COPD) or severe asthma compromise respiratory function, making the lungs less capable of clearing infection and withstanding inflammation.
Ongoing cancer treatment, specifically chemotherapy, actively suppresses the production of immune cells, leading to immunosuppression. Immunosuppression is also a consequence of medical intervention, such as in solid organ transplant recipients who take long-term immunosuppressive drugs to prevent organ rejection. These medications deliberately weaken the immune system, making the patient susceptible to common and opportunistic infections. Other conditions, like advanced HIV/AIDS, inherently weaken the immune response by attacking T-cells, which are central to adaptive immunity.
Vulnerability Based on Age and Development
Susceptibility is strongly linked to the physiological stage of life, affecting both the very young and the elderly. Infants and young children are considered highly susceptible due to their developing, or naive, immune systems. The adaptive immune system, responsible for recognizing and remembering specific pathogens, is not fully mature until the first decade of life.
Newborns rely heavily on their innate immune system and maternal antibodies transferred during pregnancy, but this protection wanes over the first few months. Their immune cells show reduced responsiveness, making them vulnerable to a wide range of bacterial and viral infections. Their smaller body size and physiological reserves also make them susceptible to complications like severe dehydration or rapid deterioration from high fever.
At the other end of the spectrum, older adults aged 65 and above are highly susceptible due to immunosenescence, the natural decline of the immune system with age. This decline affects both innate and adaptive immunity, resulting in a reduced ability to produce new immune cells and a diminished response to vaccines. The immune system often exists in a state of chronic, low-grade inflammation, which compromises its ability to respond effectively to new infections. This physiological decline is a major factor why the majority of severe illness and mortality occurs in this population.