The girl, a pupil at Ysgol Hellan in Denbigh, lives at a neighbouring property to the suspected source of the outbreak at a farm in Llanfihangel Glyn Myfyr, near Corwen, where it was confirmed that 15 chickens had died of the virus last Thursday.
Pupils and their parents at the school, who were on half-term, were called in for a series of meetings after the girl fell ill over the bank holiday weekend. Children who came into direct contact with the pupil have been given the anti-viral drug Tamiflu as a precaution.
The girl's parents have allowed officials, who do not normally comment on individual cases, to release more details of her condition because they fear she is being seen as central to the outbreak.
A spokesperson for the National Public Health Service for Wales praised Denbighshire local authority for acting swiftly.
"The school is carrying on as normal. This is simply precautionary," he said.
It is not often a director of education will break a holiday to hand out information. The girl suspected of having the virus has tested negative, but it is believed she did contract it - and the test has not picked it up - because the virus loses strength quickly.
The number of people identified as having been in contact with the virus was almost 230 when TES Cymru went to press. But those confirmed as showing signs of contamination remained unchanged at 12. Symptoms are said to be flu-like, with conjunctivitis.
A 1km exclusion zone, to prevent the movement of birds, is in place around the infected farm and 30 chickens were slaughtered last week. The H7 strain is not as potentially life-threatening as the H5N1 type that has killed 180 people in south-east Asia.