Several groups of people are of special concern and need to be targeted when considering emerging infectious diseases.

Persons whose normal host defenses have declined with age or been impaired by illness or medical treatment are more likely to become ill with opportunistic infections. Over the past two decades, the most significant emergence of opportunistic infections has been in persons infected with HIV. Other populations at risk include recipients of bone marrow and solid organ transplants, patients receiving chemotherapy or chronic steroid therapy, burn victims, patients on renal dialysis or with indwelling medical devices, newborns and the elderly.

Certain asymptomatic infections in a pregnant woman can increase the risk of prematurity, low birth weight, long term disability, or death. Some infections may be transmitted from mother to child during pregnancy, delivery, or breast feeding.

Persons who cross international boundaries (e.g. tourists, workers, immigrants, and refugees) are at increased risk for contracting infectious diseases and can also disseminate diseases to new places. International travel has increased substantially in recent years, and more travelers are visiting remote locations where they can be exposed to infectious agents that are uncommon in their native countries. Immigrants may come from nations where diseases like tuberculosis are endemic, and refugees may come from sites where crowding and malnutrition create ideal conditions for the spread of diseases such as cholera, malaria, measles, and varicella.

USDHHS (1998), Preventing Emerging Infectious Diseases: A Strategy for the 21st Century, Atlanta: Centers for Disease Control and Prevention.