Influenza virus acutely induces infection of upper and lower respiratory
tracts in humans.  Following the latency of 1 or 2 days, fever (38 to 39 ℃)
as well as headache, lumbago, joint pain, myalgia, systemic malaise and
gastrointestinal symptoms occur at high frequencies.
  The most problematic complications of influenza are elderly and cephalitis
and encephalopathy in children.
  Since cephalitis and encephalopathy in children, particular, have high fever,
consciousness disorder and convulsion as main symptoms and exhibit very
poor prognosis and the period from initial symptoms, development of central
nervous symptoms to death is very short, quick diagnosis and treatment are required.
  Influenza virus belongs to Orthomyxoviridae and has the serotypes of A, B
and C. It is said that the type A is the most common type, which shows the
most serious symptoms according to the virus strain, and the type B shows
slightly milder symptoms than the type A.
  Conventionally, the diagnosis based on the antibody titer using the paired
serum has mainly been conducted at intervals of 2 to 4 weeks, which is time-
consuming and has not been used widely for diagnosis.
  Recently, the reagent quickly detecting the influenza virus antigen was
developed and began to become popular as a diagnostic drug, and therap-
eutic drugs were also developed, and proper treatment based on quick diag-
nosis has become possible.