Influenza, more commonly known as “flu,” is a viral infection of the respiratory tract that affects the nose, throat, and sometimes lungs.

Outbreaks of flu tend to happen annually, at about the same time every year. However, each outbreak may be caused by a different subtype or strain of the virus, so a different vaccine is needed to prevent the flu each year.

For most people, a bout of flu is an unpleasant but short-lived illness. For others, however, flu can pose serious health risks, particularly if complications such as pneumonia develop.

Every year, thousands of Americans die from the flu. According to the Centers for Disease Control and Prevention (CDC), the number of deaths caused annually by flu in the United States ranged from 3,000 to 49,000 between 1976 and 2006, with an annual average of 23,607 flu-related deaths.

The best way to avoid getting the flu is to get an annual vaccination, encourage the people you live and work with to do likewise, stay away from people who are sick, and wash your hands frequently.

Types of Flu

There are three types of human influenza virus — type A, type B, and type C — and many different variants within those types.

Because several types or strains of flu can circulate simultaneously, each year’s flu vaccine protects against the three or four viruses predicted to be most common in the coming season.

Influenza A and B cause the seasonal epidemics of flu, with symptoms that can range from mild to severe. Two types of influenza A and one or two influenza B viruses are included in the seasonal vaccine.

Influenza C causes a mild respiratory illness and is not thought to cause epidemics. It is not included in the flu vaccine.

H1N1, or swine flu, is a type of influenza A that infects pigs and can also infect humans. It is currently included in the flu vaccine.

Avian flu (H5N1) naturally occurs in wild aquatic birds and can infect domestic poultry. It rarely infects humans but can be severe when it does. Most cases in humans have occurred in Asian countries.

While some people refer to a brief episode of nausea, vomiting, and malaise as “24-hour flu” or “stomach flu,” these are unrelated to influenza and are usually due to infection with other viruses such as norovirus.

How the Flu Spreads

The flu spreads when an infected person coughs, sneezes, or talks, propelling droplets of liquid into the air that either land on another person’s mouth or nose or are inhaled into a person’s lungs.

The flu may also spread through direct contact with an infected person’s nasal secretions, or by touching an object with flu virus on it, then touching one’s mouth or nose.

Symptoms of flu typically start within one to four days of infection.

An infected adult may be contagious from one day before symptoms start to 5 to 7 days after becoming sick. Children may continue to be contagious for longer than 7 days.

Given how easily the flu is transmitted, staying home while sick and keeping sick children home are important ways to reduce the spread of the flu.

Flu Complications

Complications of the flu can include:

• Asthma flare-ups
• Bronchitis
• Ear infections
• Heart problems
• Pneumonia
• Sinus infections

Complications of influenza may be caused by the flu virus itself or by a bacterial infection, called a secondary bacterial infection.

When a patient is ill, it can often be difficult to determine if their illness is from the flu alone or a secondary bacterial infection.

If the infection is caused by bacteria, antibiotics may be used to treat it. If it’s caused by the flu virus, antibiotics will not be helpful in treating it, since antibiotics have no effect on viruses.

Sources:

• Seasonal Influenza (flu); CDC.
• What You Should Know for the 2014-2015 Influenza Season; CDC.
• Influenza (flu); Mayo Clinic.
• H5N1 Avian Flu (H5N1 Bird Flu); Flu.gov.