The influenza virus is a highly contagious respiratory pathogen that causes yearly seasonal epidemics and occasional pandemics, leading to significant health and economic consequences. It spreads through droplets, aerosols, and surfaces. Infected individuals are contagious before symptoms appear, with peak infectivity early in the illness. In the United States, the 2024-2025 influenza season was the most severe since 2017-2018, with an estimated 47 to 82 million cases, up to 1.3 million hospitalizations, and up to 130,000 deaths. The risk of adverse outcomes is higher for vulnerable groups, including older adults, infants, and individuals who are immunocompromised, those with chronic conditions, and individuals who are pregnant, long-term care facility residents, and individuals experiencing homelessness. Vaccination is the main preventive measure and is recommended for all patients 6 months and older unless contraindicated. Advances in vaccines have improved efficacy. For the 2024-2025 vaccine, efficacy ranged from 32% to 78%, depending on age group and outcome. Misinformation, limited access, mistrust, and disparities are barriers to administration. Antiviral medications including oseltamivir and zanamivir can reduce illness and complications if given early in the infection course, and new therapies are being developed for drug-resistant strains. Strategies to more effectively address influenza include improving access to vaccines, countering misinformation, performing community outreach, and advancing research on antiviral medications and universal vaccines.
Naik H. Viral Respiratory Infections: Influenza. FP Essent. 2026;569:15-27.
Case 2. KB is a 69-year-old professor with controlled type 2 diabetes, hypertension, and chronic obstructive pulmonary disease who presents with 1 day of fever, chills, dry cough, headache, and myalgia. Several students in her class have had influenza. On physical examination, her temperature is 98.0°F (36.7°C), she is in no acute distress, and her lungs are clear on auscultation. She is concerned about her risk of complications given her medical conditions and asks whether she should be tested for influenza and started on treatment.
Influenza virus is a highly contagious pathogen affecting the respiratory system, including the nose, throat, and lungs. A significant portion of infected individuals have no symptoms.1 Symptoms can range from mild to severe and usually resolve in weeks, but serious complications and death can occur.
Transmission Routes
Droplet transmission of influenza occurs when an infected person coughs, sneezes, or talks, releasing tiny droplets that carry the virus through the air. These droplets cause infection when they land in the mouth or nose of individuals nearby, typically within 1.8 m (6 ft). Although droplet transmission is the primary route, some evidence suggests the virus may spread through smaller aerosolized particles that can travel farther and remain airborne for extended periods. Research in this area is ongoing.2
Another route of spreading is through contaminated surfaces, also known as indirect contact or fomite transmission. In this case, a person contracts influenza by touching a contaminated object or surface and then touching their mouth, nose, or eyes. The virus can remain infectious on surfaces for several hours. Although fecal-oral transmission is the usual route of infection in birds and has been documented in other mammals, it does not play a significant role in influenza infection in humans.3
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