Complications of the flu will often appear just as main flu symptoms are subsiding. They include:

Lower Respiratory Tract Infections

One of the biggest dangers of influenza comes not from the virus itself but from secondary bacterial infections of the lower respiratory tract. The World Health Organization estimates that each year between one-quarter and one-half million people worldwide die from complications of the flu. That number can rise to several million during worldwide pandemics.

Pneumonia

Pneumonia is the most common complication of the flu and can be especially dangerous for people in high-risk groups, such as the young, the elderly, and those with other existing conditions. The lungs become inflamed and cause symptoms of cough, fever, shaking, chills, and other side effects.

Encephalitis

Encephalitis is swelling and inflammation of the brain caused when viruses enter the brain tissue and nearby membranes. It can lead to destroyed nerve cells, bleeding in the brain, and brain damage. Symptoms include headache, fever, vomiting, light sensitivity, drowsiness, and clumsiness. Though rare, severe flu pandemics may result in a Parkinson’s like syndrome of tremor and difficulty with movement.

Bronchitis

Bronchitis is an irritation of the mucous membranes of the bronchi in the lungs, which can be acute or chronic. While symptoms like chest tightness, fatigue, shortness of breath, and other symptoms can be problematic, more serious conditions such as emphysema, heart failure, and pulmonary hypertension can develop as a result of bronchitis.

Sinusitis

Showing similar symptoms as bronchitis, sinusitis is the inflammation of the sinuses. Other symptoms include nasal congestion, sore throat, postnasal drip, and pain in the sinuses.

Otitis media

Better known simply as an ear infection, otitis media is an infection and inflammation of the ear. Symptoms include chills, fever, hearing loss, ear drainage, vomiting and irritability.

Other complications

Contracting the flu can also aggravate existing pathologies such as heart, lung, kidney, or liver disease.