Inﬂuenza is contagious, with a reproductive number estimated at 1.4–41 (see section 1.1.1). The virus is transmitted by droplets generated by sneezing and coughing that land directly on the respiratory mucous membranes, by direct or indirect contact (contaminated hands or fomites), or by the aerosol route.2
The incubation period can range from one to seven days (average one to three days), during which time the virus replicates in the ciliated columnar epithelial cells of the upper and lower respiratory tract. An infected person is contagious from one to two days before symptoms start until about day ﬁve of the illness. Peak viral shedding occurs one to three days after the development of symptoms, diminishing to low levels by ﬁve days. Children shed more virus and remain infectious for longer than adults.
In older children and adults the illness characteristically begins abruptly with fever and a variety of clinical symptoms, including chills, malaise, headache, myalgia, non-productive cough, rhinitis, sore throat and mild conjunctivitis. Vomiting and diarrhoea may be present. While children aged under 5 years have fever, cough and rhinitis, infants may present with rhinitis only.
There is a wide range of symptoms, from relatively asymptomatic to severe disease. Mild influenza is common and symptoms can be non-specific, resulting in a large proportion of undetected influenza infections.
In the young, inﬂuenza virus may cause croup, bronchiolitis and pneumonia. Fever is often less evident in the elderly. Inﬂuenza typically resolves after several days in the majority of people, although cough and malaise may persist for two or more weeks.
Infections due to pandemic influenza A strains are more likely to lead to severe morbidity and increased mortality than influenza B or seasonal influenza A strains.
In some people, inﬂuenza can exacerbate underlying medical conditions, such as pulmonary, cardiac or metabolic disease. Some of the many reported complications associated with inﬂuenza include pneumonia, respiratory failure, myositis, encephalopathy, myocarditis, pericarditis, Reye syndrome (associated with aspirin use in children), bronchitis, otitis media and death.