1.2 Influenza infection in humans

Influenza is an acute viral disease that affects the respiratory tract in humans. The virus is spread readily in aerosol droplets produced by coughing and sneezing, which are symptoms of the illness. Other symptoms include fatigue, muscle and joint pains and fever.

Following infection, the influenza virus replicates in the cells lining the host’s upper and lower respiratory tract. Virus production peaks 1–2 days later, and virus particles are shed in secretions over the following 3–4 days. During this period, the patient is infectious and the symptoms are typically at their most severe.

After one week, virus is no longer produced, although it is possible to detect viral antigens for up to 2 weeks. Immune responses are initiated immediately after the virus starts to replicate, and antibodies against the virus start to appear in the blood at 3–4 days post infection. These continue to increase over the following days and persist in the blood for many months.

In a typical flu infection, the virus is completely eliminated from the host’s system within 2 weeks. This is sterile immunity: the virus cannot be obtained from the patient after recovery from the disease. Figure 2 shows the typical time course of an acute flu infection.

Figure 2 Time course of a typical flu infection. Production of the virus (purple bar) starts early after infection and is maximal within two days. The infection is contained during this period by various immune defences. Specific antibody production starts to appear by day 3 (yellow bar), and this contributes to the elimination of the virus. Symptoms (red bar) coincide with the production of the virus.

Show description|Hide description

In this figure, the horizontal axis is labelled days and is marked from zero, which is the time of infection, up to 10 days at one-day intervals. There are three horizontal bars, labelled virus, symptoms and antibodies, and the depth of each bar corresponds to the amount of virus or antibody or the severity of symptoms respectively. Antibody production is maximal by day 5 and the virus is reduced to a very low level by day 6. Symptoms of infection disappear by day 9.

Figure 2 Time course of a typical flu infection. Production of the virus (purple bar) starts early after infection and is maximal within ...

For infants, older people, and those with other underlying diseases (e.g. of the heart or respiratory system) an infection with flu may prove fatal. However, the severity of a flu epidemic and the case fatality rate depend on the strain of flu involved and the level of immunity in the host population.

During a severe epidemic, there are typically thousands more deaths than would normally be expected for that time of year, and these can be attributed to the disease. Although older people are usually most at risk from fatal disease, this is not always so. In the 1918 flu pandemic there was a surprisingly high death rate in people aged 20–40 (Figure 3), and this was also the case for the 2009 ‘swine flu’ pandemic.

Figure 3 Mortality according to age in the 1918 USA flu epidemic: 1917 (red) and 1918 (blue) rates in males and females. This epidemic was notable because it particularly affected people aged 20–40 years.

Show description|Hide description

The figure is a line graph in which the horizontal axis is labelled age and is marked in years from zero to 80 at intervals of 20 years. The vertical axis is labelled death rate per 100 000 population and is marked from zero to 3000 at intervals of 500 units. The 1917 plots for both males and females are almost identical U-shaped curves, and show that death rates were in excess of 1500 in newborns, dropped to 200 by age 2 and were zero in 10-year-olds. Rates remained below 200 for those aged 40 or under, increased to 500 in 65-year-olds, and beyond this age, death rate increased very rapidly to reach levels of 3000 per 100 000 for 80-year-olds. The pattern in death rates in 1918 for the younger and older age groups was similar to that in 1917, although rates in newborns were much higher, at 2500. The minimum rate in 1918, again observed in 10-year-olds, was 200. Beyond this age, death rates rose sharply, to 1200 for 30-year-old males and 750 for 30-year-old females. Death rates in both groups fell rapidly, and for those aged 60 or more, were similar to the rates in the previous year.

Figure 3 Mortality according to age in the 1918 USA flu epidemic: 1917 (red) and 1918 (blue) rates in males and females. This epidemic ...

Older people are often most severely affected during infectious disease outbreaks because they may have a less effective immune response than younger people, or a reduced capacity to repair and regenerate tissue damaged by the infection. However, there are circumstances where older people may be more resistant to infection than younger people because they may have already encountered the disease (in their youth) and could retain some immunity and so be less susceptible than younger people who have not encountered the disease before.