These include fever, shivering, chills, malaise, dry cough, loss of appetite, body aches, nausea, and sneezing typically in connection with a sudden onset of illness.

[1] In most cases, the symptoms are caused by cytokines released by immune system activation,[citation needed] and are thus relatively non-specific.

[citation needed] The term ILI can be used casually, but when used in the surveillance of influenza cases, can have a strict definition.

The World Health Organization defines an illness as an ILI if the patient has a fever greater than or equal to 38 °C and a cough, which began in the last 10 days.

If the patient requires hospitalisation, the illness is classified instead as a severe acute respiratory infection (SARI).

[citation needed] In most years, in the majority of samples tested, the influenza virus is not present (see figure above).

[8] Infectious diseases causing ILI include respiratory syncytial virus, malaria, acute HIV/AIDS infection, herpes, hepatitis C, Lyme disease, rabies, myocarditis,[9] Q fever, dengue fever,[10] poliomyelitis, pneumonia, measles, SARS, COVID-19, and many others.

Pharmaceutical drugs that may cause ILI include many biologics such as interferons and monoclonal antibodies.

[4] The use of multiplexed point-of-care testing such as CRP (C-reactive protein) along with an examination by a doctor may help to identify a bacterial and avoid an unnecessary antibiotic prescription.

"[17] A diagnosis of confirmed swine flu required laboratory testing of a respiratory sample (a simple nose and throat swab).