The germs that cause the flu are spread when an infected person sneezes or coughs without covering his or her nose and mouth.⁶ The tiny droplets that are released from the body upon sneezing or coughing contain the flu germ that makes others sick.⁶ When a healthy person breathes in the droplets, the flu virus causes him or her to become sick within a few days.⁴ A healthy person can also get sick if he or she touches something with the germs on it, and then touches his or her eyes, nose or mouth.¹⁵

When the virus enters the body, it invades the lining of the throat and nose to quickly cause infection.⁴ The virus then copies itself and causes the other healthy cells within the body to become sick with the flu virus.¹⁶ Common symptoms include fever⁴, sore throat⁶, cough¹⁶, body aches⁵ and headache.⁴

Because influenza virus is airborne, it is easily transmitted, placing many at risk of exposure to influenza, including groups such as school-aged children and adults in frequent contact with children, such as parents, teachers and childcare providers.

Emergency Warning Signs of Flu in Children

Seek immediate medical attention if your child shows any of the following symptoms, as they may be a sign of complications from the flu:¹⁷

• High and prolonged fever (102°F or above for more than 72 hours)
• Bluish or gray skin color
• Drop in body temperature (hypothermia)
• Difficulty with breathing
• Not able to take in usual amount of fluids
• Changes in mental condition, such as not waking up or not interacting; being so moody that the child does not want to be held; or seizures
• Flu-like symptoms improve but then return with fever and worse cough
• Worsening of underlying medical conditions (for example, heart or lung disease, diabetes)

Emergency Warning Signs of Flu in Adults

Adults should seek immediate medical attention if they exhibit any of the following symptoms, as they may be a sign of complications from the flu:¹⁷

• High or prolonged fever (104°F or above or a temperature of 101°F that persists for more than 3 days)
• Difficulty with breathing
• Pain or pressure in the chest
• Near-fainting or fainting
• Confusion
• Severe or persistent vomiting

Virus Types and Antigenic Drifts

There are several types of flu viruses.⁴ Influenza types A and B are responsible for the debilitating symptoms associated with the flu.⁴

The flu virus is constantly changing, disguising itself through mutation to avoid the body's immune system.⁶ The gradual buildup of mutations, or "antigenic drift," leads to the introduction of new flu virus strains from year to year.⁶ Therefore, antibodies developed by the human body against current flu viruses may not provide protection against new strains.⁶ Although infrequent in occurrence, antigenic shifts cause flu viruses to emerge that are either new or have not circulated in many years.⁶ When antigenic shift happens, the flu virus can spread very rapidly from one person to another, causing a pandemic.⁶

A flu pandemic is similar to a flu epidemic but occurs on a global scale. The most catastrophic flu pandemic on record was the 1918-1919 "Spanish flu," which resulted in more than 20 million deaths worldwide.⁵ Most of these deaths were in young and otherwise healthy adults.¹¹

The most recent severe pandemic occurred in 1968, and scientists believe that it is inevitable that a new pandemic strain will emerge in the future.¹⁸ Given the trend of increased mobility and urbanization of the rapidly expanding population, people in the 21^st century could be especially vulnerable during such an outbreak.¹¹

For these reasons, it is important to be vaccinated annually against the flu (when the nation’s supply of vaccine is available).⁶ Each year, flu vaccines are made to match the three most prominent influenza strains predicted for the coming flu season.⁶ Vaccination introduces the antigens of those viral strains to the body, stimulating antibodies that help the body to resist infection.¹⁶