The causes of<unk>  range from benign self-limited<unk> es such as gastroenteritis, rhinoviral disease, and<unk> , to severe, sometimes life-threatening, diseases such as meningitis, sepsis, and leukemia.<unk>  Technically, any clinical diagnosis of<unk>  is a diagnosis of<unk> , not of<unk> . This distinction usually is of no great concern because, regardless of cause, most cases of<unk>  are mild and self-limiting. Furthermore, except perhaps during the peak of a major outbreak of<unk> , most cases of<unk>  are not due to<unk> .<unk>  is very common: in the United States each adult can average 1–3 episodes per year and each child can average 3–6 episodes per year.<unk>  in humans is subject to clinical surveillance by a global network of more than 110 National<unk>  Centers. These centers receive samples obtained from patients diagnosed with<unk> , and test the samples for the presence of an<unk>  virus. Not all patients diagnosed with<unk>  are tested, and not all test results are reported. Samples are selected for testing based on severity of<unk> , and as part of routine sampling, and at participating surveillance clinics and laboratories. The United States has a general surveillance program, a border surveillance program, and a hospital surveillance program, all devoted to finding new outbreaks of<unk> . In most years, in the majority of samples tested, the<unk>  virus is not present (see figure above). In the United States during the 2008–9<unk>  season through 18 April, out of 183,839 samples tested and reported to the CDC, only 25,925 (14.1%) were positive for<unk> . The percent positive reached a maximum of about 25%. The percent positive increases with the incidence of infection, peaking with the peak incidence of<unk>  (see figure). During an epidemic, 60–70% of patients with a clear<unk>  actually have<unk> . Samples are respiratory samples, usually collected by a physician, nurse, or assistant, and sent to a hospital laboratory for preliminary testing. There are several methods of collecting a respiratory sample, depending on requirements of the laboratory that will test the sample. A sample may be obtained from around the nose simply by wiping with a dry cotton swab. Other causes Infectious diseases causing<unk>  include respiratory syncytial virus, malaria, acute HIV/AIDS infection, herpes, hepatitis C, Lyme disease, rabies, myocarditis, Q fever, dengue fever, poliomyelitis, pneumonia, measles, SARS, COVID-19, and many others. Pharmaceutical drugs that may cause<unk>  include many biologics such as interferons and monoclonal antibodies. Chemotherapeutic agents also commonly cause flu-like symptoms. Other drugs associated with a flu-like syndrome include bisphosphonates, caspofungin, and levamisole. A flu-like syndrome can also be caused by an<unk>  vaccine or other vaccines, and by opioid withdrawal in physically dependent individuals.
Influenza-like illness
Anaphylaxis
Intermittent fever
Asymptomatic
Proteus-like syndrome
Bilious fever
Morning sickness
Fever of unknown origin