An influenza epidemic that killed at least 1 million worldwide nearly 40 years ago was largely shrugged off in Hong Kong because most people were young and healthy, says a former health chief who was at the centre of the medical response. But an outbreak of bird flu now could be deadly because the population is older and the strain is more virulent.

Lee Shiu-hung recalls the 'Black Saturday' in 1968 when Hong Kong flu was first recognised. Estimates of the number of people who succumbed to the virus before it ended in 1969 range up to 4 million, according to the World Health Organisation.

'It was July 13 when we realised we had recorded about 300 patients [with flu] at our 11 public clinics within a week,' Professor Lee said. 'The weekly number jumped to 3,500 two weeks later. Our clinics were so busy that many department staff - including myself who worked in the department head office - had to see patients in our public clinics, which extended their service until about 10pm.'

Professor Lee, now 72, joined the government as a junior doctor in 1960. He was a principal medical officer at the time of the outbreak. He would go on to lead the health department from 1989 to 1994.

'But it did not show a full picture because we did not have a surveillance system at the time to track the situation for private doctors,' he said.

'In fact, the general health conditions of patients were not too serious and not many people needed hospital treatment. It was mainly because the majority of the population was young people who had stronger immunity.'

The epidemic was the first known outbreak of the H3N2 strain. It infected 10 per cent of the population, but killed only 27 people in Hong Kong, mainly very young and very old patients, according to government data.

'Of course, we did not have flu shots or Tamiflu at that time. What we provided to patients were symptomatic treatments such as cough mixture for those with cough, and anti-fever medicine if patients developed fever ... or we used antibiotics in the case of pneumonia. We did not have any special isolation facilities in the clinics at that time.

'What we did mainly was to step up public education to promote personal hygiene, such as washing hands and staying away from crowds. For patients, we suggested that they not keep too close to other people to avoid spreading [the virus]. All these are the golden rules which the Department of Health is suggesting people follow,' Professor Lee said.

'As a doctor, I did not think of any risk at the time and did not even wear a mask when seeing patients.'

Professor Lee said a bird flu outbreak would be more deadly partly because of the ageing population, with 980,000 people, or 14.6 per cent of the population, over 60. He said that other major challenges came from drug resistance, easy and widespread travel, and urbanisation.

'Urbanisation also plays a key role in viral mutations as our contacts with animals have become too close', raising the chances of the virus undergoing genetic mutation. 'All these would make the diseases nowadays more virulent and wide-spreading than 40 years ago.'

Professor Lee said that whether Hong Kong could protect itself from bird flu infections heavily depended on the measures and joint efforts taken at different levels - personal hygiene of individuals, joint efforts by medical experts and co-operation between governments. He described central slaughtering of poultry as a 'step in the right direction' in protecting humans from bird flu.