Level 6, meaning a pandemic has begun, is defined as "efficient and sustained" human transmission.

However, to date, there has been no evidence of widespread or sustained human-to-human transmission in Indonesia.

It was showing "sustained human-to-human transmission", thereby justifying the WHO's pandemic phase 5 rating.

Phase four is characterized by sustained human-to-human transmission that allows community-level outbreaks, and indicates that a pandemic is possible.

These types of networks would be best used in short durations for bursts of traffic rather then sustained transmissions.

So far, this strain of flu has not had any sustained human-to-human transmission.

So far there has been no sustained human-to-human transmission, but that is what health officials fear, because it could cause a pandemic.

"You probably need a group of people, with sustained transmission," said Dr. Michael Martin, an epidemic intelligence service officer at the centers.

The first sustained human-to-human transmissions might start insidiously, perhaps within one family whose infections aren't immediately identified as H5N1.

Avian flu cannot yet be categorized as a "pandemic", because the virus cannot yet cause sustained and efficient human-to-human transmission.