The terms an epidemiologist use

The Asian Flu Pandemic was an outbreak of avian influenza that started in China and spread worldwide.

The estimated death rate was one to two million people.

Typhus fever is spread by lice. The war brought on poor sanitation that probably led to a higher density of lice and made the transmission more prevalent.

Typhus fever caused three million deaths in Russia alone.

The earliest recorded pandemic happened during the Peloponnesian War. It passed through Libya, Ethiopia, and Egypt,  and it reached Athens as the Spartans laid siege. Two-thirds of the population died.
The disease, suspected to have been typhoid fever, weakened the Athenians significantly and played a big role in their defeat by the Spartans.

It may have been an early appearance of smallpox that began with the Huns.
The Huns then infected the Germans, who passed it to the Romans and then returning troops spread it throughout the Roman empire.  This plague continued until about 180 A.D., claiming Emperor Marcus Aurelius as one of its victims.

When declaring a pandemic, the World Health Organization has the last word. There is no threshold, such as a certain number of deaths or infections, or a number of countries affected, that needs to be met.
And once a pandemic is declared, it becomes more likely that community spread will eventually happen, and governments and health systems need to ensure they are prepared for that.

Although a pandemic has been declared, there is no need for global panic. Panic would defeat the purpose of trying to raise awareness.
It is still urging countries to detect, test, treat, isolate, trace and mobilize their people.

Staying home during the pandemic helps prevent health systems from being overloaded.
Overloading hospitals can likely be averted with protective measures like closing schools, canceling mass gatherings, working from home, self-quarantine, self-isolation, avoiding crowds. All these keep the virus from spreading fast.

• The original use of the word Quarantine was the act of anchoring a ship arrived in Venice, Italy, for 40 days before landing.
• Infectiousness of any disease is measured by the reproduction number (R0, or R naught). For example, Smallpox has an R0 of 6 whereas Measles has an R0 of 16. 
• Vaccination, if available, and herd immunity can curb the spread of disease.
• Big cities, with exploding population and traffic, can lead to the rapid spread of any infectious disease.

From April 12, 2009 to April 10, 2010, CDC estimated there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (range: 195,086-402,719), and 12,469 deaths (range: 8868-18,306) in the United States due to the (H1N1)pdm09 virus.
Additionally, CDC estimated that 151,700-575,400 people worldwide died from (H1N1)pdm09 virus infection during the first year the virus circulated.

Though the 2009 flu pandemic primarily affected children and young and middle-aged adults, the impact of the (H1N1)pdm09 virus on the global population during the first year was less severe than that of previous pandemics.
The (H1N1)pdm09 virus was very different from H1N1 viruses that were circulating at the time of the pandemic. Few young people had any existing immunity (as detected by antibody response) to the (H1N1)pdm09 virus, but nearly one-third of people over 60 years old had antibodies against this virus, likely from exposure to an older H1N1 virus earlier in their lives.

The so-called Spanish flu did not originate in Spain. The geographic origin of the flu is debated to this day, though hypotheses have suggested East Asia, Europe, and even Kansas.
The influenza pandemic from 1918 got this name most likely because of the WWI context: The major countries involved in the war were keen to avoid encouraging their enemies, so reports of the extent of the flu were suppressed in Germany, Austria, France, the United Kingdom and the U.S. By contrast, neutral Spain had no need to keep the flu under wraps. That created the false impression that Spain was bearing the brunt of the disease.

The 1918 flu spread rapidly, killing 25 million people in just the first six months. This led some to fear the end of mankind and that the whole thing was caused by a form of super-virus.
Recent studies show that the high death rate can be attributed to crowding in military camps and urban environments, as well as poor nutrition and sanitation, which suffered during wartime.

Focussing on the handwashing message at the most significant 10 airports in the world could reduce the spread of disease significantly at a global level (potentially by almost 70 percent).
Even small improvements in hygiene could make a huge difference. This could potentially be achieved through education, awareness, social-media nudges, public announcements and improved access to handwashing facilities.

“Save the economy or save lives” is a false choice.
A group of economists published a paper on the 1918 flu outbreak. Their findings revealed:

• Early and aggressive interventions saved lives and triggered a faster rebound, such as job growth and banking assets.
• Without a healthy population, there can be no healthy economy.

The hope is for a deep, short recession, to show that people have shut the economy down to limit the spread of disease.

Asking millions of able-bodied workers to stop working creates a crisis of unemployment.

During this time, the U.S. is expanding unemployment benefits and are also delaying tax filing. In northern-European countries, the government is directly paying businesses to maintain their payrolls to avoid mass layoffs and furloughs.

Doctors started wearing the first surgical masks in 1897. The masks were not designed to prevent airborne disease - that is still not the case today - but to prevent doctors from coughing or sneezing droplets onto wounds during surgery.

During 1920, a plague broke out between a shared jurisdiction of China and Russia. The Chinese Imperial Court brought in a young doctor named Lien-teh Wu that determined that the plague was not spread by fleas but through the air. He expanded upon the surgery masks he'd seen in the West, and made it from gauze and cotton and added several layers of cloth to filter inhalations.

When the Spanish flu arrived in 1918, the mask was well-known among scientists and the public.