As a person whose degree is in a relevant field, I can most definitely confirm that the "Spanish Flu" has not been forgotten at all. In fact, it remains one of the main reasons why epidemiologists around the world are incredibly vigilant towards outbreak of "influenza-like" illnesses such as SARS. The threat of another global pandemic is simply too great to ignore and explicitly forms part of the WHO position on influenza.

It was emphasised over and over in my epidemiology lectures that the combination of infectionousness and mortality was what made a disease "deadly". For example, the various Ebola strains (and Rabies as well) have unbelievably high mortality but they are not easily transmitted from human to human. By contrast, the 200-some viruses that are collectively called the common cold are ridiculously transmissible but really don't kill many people at all. Influenza is in a sort of sweet spot in that it's quite infectious and moderately lethal - both of which are compounded by influenza's proven animal reservoirs (birds & pigs mainly) and ability to undergo antigenic drift and shift to present a novel virus to our immune systems.

All of which is known by medical professionals but the general public insists on juxtaposing the words "cold" and "flu". I always raise a highly skeptical eyebrow when colleagues tell me that they had the flu last week. A real bout of influenza can leave a healthy adult bed-ridden for a week or more.

Experts aside, you are correct that the popular culture has little awareness of the disease except to perhaps recall its name. The principal reason for this is also the reason why it is called the Spanish Flu and not the American (for the first recorded case) or Chinese (where epidemiologists believe the disease originated) Flu. It all has to do with reporting of casualties and deaths during the end and aftermath of WWI. Most nations, including Great Britain and America, had strict policies regarding reporting of such numbers while neutral Spain did not. That led to Spain correctly reporting a huge number of cases and a correspondingly high death toll while other nations remained silent. Hence the "Spanish Flu" name stuck.

The thing is, absent the Black Death of the 14th century, most pandemics have been forgotten except by historians and perhaps the local population. For example, there have been at least 3 major outbreaks of bubonic plague throughout history and yet most western history classes only focus on the 14th century version despite the consensus that the 5th 6th century version was at least as catastrophic and that all three versions led to major political upheaval in the territories afflicted.

One of the better known epidemics killed less than 1,000 people (Cholera, London, 1860) but is world-famous because of the efforts of one Dr John Snow who is generally regarded as the father of modern epidemiology. Unlike his TV namesake, Dr Snow did know many things and he was able to pioneer techniques to trace the source of outbreaks and in particular to trace the source of the cholera outbreak to a water pump in a certain district of London.

From here on, the reply is more speculative and I will remove this section if the mods request.

Historical epidemiology is difficult. It is nearly impossible to determine the precise nature of any given disease or epidemic based upon the descriptions of symptoms given by physicians and medical experts of the time. This is because they lacked access to modern testing equipment, were not clinical in their descriptions of symptoms and did not have a wide range of case histories to compare isolated outbreaks to.

However, in major outbreaks (such as the plagues of 5th 6th & 14th centuries), so many people were afflicted and so many people died that the description of symptoms assumes a certain uniformity that allows modern researchers to have confidence that the disease reached a certain town or traveled a certain route. This is in part because bubonic plague (of any form) displays very distinctive symptoms.

The same is not true for other pandemic diseases such as influenza, cholera, typhus, tuberculosis etc. Many of them present with strikingly similar symptoms such as coughing (bloody or not), flux (bloody or not) and fever. They are further complicated because of the pronounced tendency for complications in the absence of modern medicine and antibiotics ie. A major complication of the "Spanish Flu" was pneumonia which is readily treatable in the modern age. All of these things combine to blur the edges of a historical epidemic and to make such epidemics almost impossible to identify as coming from any given root cause.

EDIT: 5th to 6th because my idiot brain insists on calling the years of 500-599 the 5th century

Summary

The legacy of the Spanish Flu lies mainly in the attitudes and actions of modern medical professionals when confronted by the possibility of another similar global influenza pandemic.

The general public's attitudes are influenced by factors such as:

1. Countries afflicted by the Spanish Flu largely did not report their casualties due to information blackouts imposed by the hostilities in WW1
2. Due to #1, the actual extent of the disease was not identified until many years after the last recorded case
3. The general public often does not view influenza as a serious illness. This opinion is not shared by epidemiologists
4. The Spanish Flu underpins much of the WHO (and similar bodies) positions on the seriousness of influenza
5. It is very difficult to identify epidemics without the benefit of modern epidemiological techniques. Such epidemics quickly fade from the memory of the general public

Sources:

• Graduate level epidemiology classes :)

• Various scientific papers referred to in those classes

For the layperson:

• Justinian’s Flea: Plague, Empire, and the Birth of Europe by William Rosen

• Spillover: Animal Infections and the Next Human Pandemic by David Quammen

• The Ghost Map: The Story of London's Most Terrifying Epidemic--and How It Changed Science, Cities, and the Modern World by Steven Johnson