Anyone trying to tell themselves that major flu outbreaks are never as bad as they're cracked up to be faces a considerable obstacle: the flu pandemic of 1918-19.

Caused by a virus related to the current one, its potent ability to infect and kill was unparalleled. At least 50 million died, among them 10,651 Irish people and 228,000 Britons; between a third and half of everyone on the planet caught it; deaths were reported from Alaska to Western Samoa (where 30% of all adult males were wiped out), and, in just 18 months, it lowered the life expectancy of the average American by 10 years. No wonder the US Centers for Disease Control, an organisation not normally given to hyperbole, called it "the mother of all pandemics".

That phrase has a deliberate ambiguity, for not only did the 1918-19 virus trigger the worst of the 31 flu pandemics that have occurred since 1580, but every influenza A outbreak ever since has been caused by its descendants. Its origin, however, remains unknown: Chinese labourers out of Hong Kong who came to dig war-time trenches in Europe; a spontaneous eruption in Austria; something incubated on the plains of Kansas, the battlefields of the Middle East, or in the heat of Sierra Leone – science has never pinned it down.

Initial reports of a flu-like infection came from the Western front, where soldiers suffered what was called "three-day fever", a name that was soon to change. The first place the illness attracted widespread comment was Madrid – not at war, and so lacking press censorship ? where the royal family was stricken. Thus it was christened 'Spanish Flu' (a libel which Spaniards repaid by calling it 'French Flu').

There were three overlapping waves. The first, in spring and summer 1918, was the least deadly, although troublesome enough. In May that year, the British navy was confined to port for three weeks because more than 10,000 sailors were sick, the army's 29th Division had so few men fit that it had to cancel an attack, and German forces were even harder hit, with Field Marshall Ludendorff later blaming the flu for his nation's defeat. By early summer, it had reached British cities ? Glasgow was the first ? and it was in London by June.

At this stage, the flu was highly infectious, but not especially deadly. That was to change in early autumn. By 21 October, nearly 4,000 Britons were dying every week, and schools and half London's theatres were closed.

British precautions, however, never reached the ambition of public officials in the US, where Kansas City banned weddings and funerals attended by more than 20 people, Seattle and San Francisco decreed the wearing of face masks compulsory, and New York and Chicago made it a crime not to catch your sneeze or cough in a handkerchief.

By the first week of November, 1,000 Britons were dying every day, doctors and nurses were in pitifully short supply, and police, fire, and other public services were severely hit. In some places, there were barely enough able-bodied living to bury the dead.

The flu proved, as second wave turned to third in 1919, to have a mortality rate many times greater than ordinary flu. It could also kill with astonishing speed. There were reports of people hale at breakfast and dead by tea-time.

And it was especially lethal, not – as is normal ? among infants and elderly, but among 20-40 year olds. Their vigorous immune systems reacted so comprehensively to the virus that the lungs of worst cases filled with blood and other fluids, and the victims 'drowned'.

But for all its hideous detail, and similarity to the present virus, the 1918-19 pandemic is by no means a portent. It struck when medical knowledge and skills were 90 years less advanced than now. The flu virus was not isolated until 1933, and antibiotics were unknown.

In 1918, there was not even a Ministry of Health in Ireland or Britain. Diagnostics were so poor (most doctors working then would have been trained in the 19th century) that patients were confidently told they had cholera, encephalitis, or an illness caused by eating too many turnips.

We have - we hope - moved on a little since then.