Purdue News Service

Stages of alert system

Stage one means there is no new virus subtype detected in humans, and the risk of human infection is considered to be low, but a flu subtype that has caused human infection may be present in animals.

At stage two, there is a higher risk of human cases due to the circulation of the animal flu virus, but no new virus subtypes have been detected in humans.

Stage three, four and five are considered the pandemic alert period.

Stage three means a new subtype is causing human infections, but it is not yet spreading efficiently and sustainably.

Stage four means there is evidence of human-to-human transmission, but it is limited and localized.

At stage five, there is evidence of significant human-to-human transmission, suggesting that the virus is becoming better adapted to humans.

The final stage, stage six, is called the pandemic period. At this point, there is increased and sustained human-to-human transmission.

The world is presently in stage three. The main public health goal now is to concentrate on early detection, notification and response to additional cases.