Influenza-like illness (ILI) is defined as a temperature of > 100.0?F (> 37.8?C) and either cough or sore throat in the absence of a known cause. Levels of influenza activity are 1) no activity, 2) sporadic-small numbers of laboratory-confirmed influenza cases or a single influenza outbreak reported but no increase in cases of ILI, 3) local-outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of a state, 4) regional-outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least two but less than half the regions of a state, and 5) widespread-outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of a state.
Influenza incidence exhibits strong seasonal fluctuations in temperate regions throughout the world, concentrating the mortality and morbidity burden of the disease into a few months each year. Influenza is more likely to spread in the winter than the summer. This may be caused by an increased infectiousness of the disease, an increased susceptibility of people, or an increased number contacts with others that might result in transmitting the infection during the winter. For example, people may spend more time indoors.
The reason that more people catch the flu in the winter appears to be that small seasonal changes in flu transmission at the individual level are greatly amplified as the disease spreads through communities. The underlying cause of seasonal fluctuations in transmission may be too small to measure. Large fluctuations in the number of flu cases between winter and other seasons may be caused by very small changes in the number of people infected by a single infectious person. These small changes in transmission rate are amplified by interactions between the evolving virus and the changing level of immunity that people have to specific strains.
The cause of influenza's seasonality has remained elusive. Studies have failed to establish whether these transmission changes are due to direct effects of temperature and humidity on transmission, to changes in mixing patterns [e.g., school terms or simply more time spent indoors], or to other factors, such as increased viral production under winter conditions. In fact, it may be impossible to establish the underlying cause of seasonality in influenza epidemics, since the large observed oscillations in incidence can be generated by seasonal changes in the transmission rate that are too small to measure. The large oscillations in incidence may be caused by undetectably small seasonal changes in the influenza transmission rate that are amplified by dynamical resonance.
Data can be evaluated quantitatively, and graphic representation of this information, known as epidemic curves (epi-curves), may prove especially useful in this endeavor. These visual representations depict case frequency over time, and are initially used to obtain tentative answers to questions concerning origin, propagation, incidence, prevalence, and likely modes of transmission. The nature of the epidemic curve varies with the pathogen. The frequency curve for most infectious diseases resembles a logarithmic normal curve. Epidemics such as infuenza have distinctive patterns of initiation and spread.