One teenager's determination to play soccer despite being unwell has left a web of contacts waiting to find out whether they have been exposed to the new swine flu virus.

The North Shore soccer enthusiast's innocent activity at the weekend after returning from a Rangitoto College trip to Mexico is one of many avenues through which the virus may now be spreading throughout New Zealand.

The soccer player's mother, who asked that the family not be identified, said yesterday they consulted a GP on Saturday about her son's mild respiratory symptoms, but were not told to stay home.

Only late that night, when public health nurses told them nose and throat swabs would be taken, did it become clear how serious it was.

By then the boy had played an afternoon under-17 first division match for his East Coast Bays team against Oratia United.

The web of possible exposure expanded when a substitute in that match then played, for his own East Coast Bays team, against North Force at Whangarei on Sunday in the under-17 metropolitan league.

Yesterday, Rangitoto College sent home five boys who played football with one of the students who was tested for swine flu and is now in quarantine.

Others who may have unwittingly exposed the country to swine flu include the passengers and crew on NZ1 - the Air New Zealand flight from Los Angeles which arrived in Auckland at 5am on Saturday carrying the 22 Rangitoto students and three teachers.

Fourteen Northcote College students, two parents and two staff who travelled to Mexico have been asked to stay at home.

The families of three possibly infected people were also asked to stay at home.

Results of initial tests on those three people were expected yesterday but were not known last night.

Health authorities say they were notified by two doctors about midday on Saturday of the first case of illness among the Rangitoto group.

This led to the wider investigation and subsequent tests and home isolation of the group and their families.

The teenager's case shows how difficult it is to prevent the normal social contact which can cause the spread of an infectious disease, especially at the start of an outbreak

It will not be known until today at the earliest - when results are returned from a World Health Organisation laboratory in Melbourne - if 10 people who were on the trip and have been confirmed as having influenza A, have the new strain of swine influenza.

Rangitoto College Principal David Hodge said his 3050-student school would take every possible precaution against spreading the potentially deadly disease.

Parents had been asked to keep their children home from school if they had been in close contact with any of the 25 people from the trip, and nurses at the school health centre had also screened some pupils.

Mr Hodge said two of the students who went to Mexico had the flu before they left, and one was among the students tested for swine flu.

About 100 people are in quarantine as they wait for the Melbourne laboratory test results.

Auckland Regional Public Health Service clinical director Dr Julia Peters said that if the service had been told - in line with outbreak-prevention protocols- that people on the flight had flu-like symptoms, health officials would have met the flight.

"Then nobody would have gone and played soccer."

But she did not know what the flight crew had been told "and people don't always declare themselves to them".

Air NZ said last night the crew did not know people on board had flu symptoms.

Its group general manager for international airline services, Ed Sims, said the 18 crew and two staff travelling for work purposes on NZ1 on Saturday had been told to stay at home for up to seven days in accordance with Ministry of Health standard guidelines.

In Mexico, health officials say the new strain has killed more than 80 and infected about 1400 people.

Cases have also been reported in the United States, Canada, Spain and France.

In New Zealand, health officials are trying to contact the 364 passengers on Saturday's NZ1 flight.

All will be offered the flu medicine Tamiflu and asked to stay at home for several days.

The Ministry of Health's director of public health, Dr Mark Jacobs, said last night he did not yet know how many people who had visited Mexico recently were developing symptoms, but a growing number of cases were coming to light as the message got out about the seriousness of the situation.

"We are getting a sense that there will be numbers of other people, beyond the Rangitoto College and Northcote College groups, who are considered to be possible cases and need to be tested and given Tamiflu."

Health officials are now screening all passengers arriving in New Zealand from North America.

The ministry wants anyone who travelled to Mexico or North America in the past fortnight contact to Healthline on (0800) 611-116 for information. It says they should seek medical advice if they have symptoms such as fever, cough, sore throat, runny nose, muscle aches and fatigue.

Dr Peters said the Auckland public health service had been flooded with calls from people who were on the same flights as the college students.

One of the Rangitoto teachers on the trip was admitted to hospital yesterday morning because her symptoms were not subsiding, but was discharged later in the day.

The other teacher, Charlotte Calvert, said it was a "wonderful trip" and she did not feel too unwell despite being placed in quarantine while she waited for her results to come through from Melbourne.