The following persons are increased risk for complications from influenza:

• persons aged >65 years;
• adults and children who have chronic disorders of the pulmonary or cardiovascular systems, including asthma;
• adults and children who have required regular medical follow-up or hospitalization during the preceding year because of chronic metabolic diseases (including diabetes mellitus), renal dysfunction, hemoglobinopathies, or immunosuppression (including immunosuppression caused by medications or by human immunodeficiency virus [HIV]);
• children and adolescents (aged 6 months – 18 years) who are receiving long-term aspirin therapy and, therefore, might be at risk for experiencing Reye syndrome after influenza infection;
• women who will be pregnant during the influenza season; and
• children aged 0 – 23 months.

For adults aged 65 and older, influenza can trigger complications or worsen existing health conditions, such as asthma or congestive heart failure. Bacterial infections such as pneumonia, bronchitis, and sinus and ear infections may develop just when you feel like you are getting better. These infections, particularly pneumonia, take advantage of your weakened lungs. Pneumonia is an infection of the air sacs of the lung that causes the sacs to become clogged with pus and mucus. Also individuals with chronic disorders of the lungs or heart, and cirrhosis are most at risk for developing pneumonia. In patients with cardiovascular and pulmonary disease, the death toll during epidemics from influenza or pneumonia can be as high as 870 per 100,000 population. In the group aged 50 – 64 years there is an increased prevalence of persons with high-risk conditions.

People with diabetes face a higher risk for influenza and its complications, including pneumonia, possibly because the disorder neutralizes the effects of protective proteins on the surface of the lungs. In fact, deaths among people with diabetes increase by 5% to 15% during influenza epidemics and they are six times more likely to be hospitalized with complications from influenza than non-diabetics who have influenza.

In patients whose host-defense mechanisms are compromised, infections range from minor to fatal. They often are caused by organisms that normally reside on body surfaces. Infections in this setting occur if host-defense mechanisms have been altered by age, burns, neoplasms, metabolic disorders, irradiation, foreign bodies, immunosuppressive or cytotoxic drugs, corticosteroids, or diagnostic or therapeutic instrumentation. Patients receiving corticosteroid treatment (especially in high dose) for rheumatoid arthritis, ulcerative colitis, asthma, or other diseases have an increased susceptibility to infection. Diseases such as leukemia, aplastic anemia, Hodgkin's disease, myeloma, and HIV infection are characterized by defects in host resistance. Patients with hypogammaglobu-linemia or macroglobulinemia tend to develop pneumonia. Vaccination can prevent influenza.

Children under 1 year old have a very high risk, not only for pneumonia but also for other complications, including meningitis and encephalitis (inflammations in central nervous system). The risk declines after age one but is still elevated in children aged three to five. It is often difficult to tell whether pneumonia in small children is related to influenza or caused by respiratory syncytial virus (RSV), the major viral cause of infant pneumonia. Bronchiolitis, an inflammation of the small air passages (bronchioles), is the leading cause of serious lower respiratory illness in infants. It usually affects children under the age of 2 years and is usually caused by a virus. Experts estimate that about 25% of severe lung infections are due to influenza.

Pregnant women appear to be at increased risk of hospitalization from influenza, as do other persons of any age whose ability to cope with a chronic condition is compromised by influenza infection. Pregnancy affects the mother's immune system and also puts added stress on her cardiovascular system. For this reason, pregnant women are at increased risk of complications, such as pneumonia, from influenza.