Influenza or flu is a highly contagious respiratory disease caused by influenza viruses. It is spread from one person to another through tiny droplets that are released into the air when a person infected with the flu coughs or sneezes.. Winter is the worst season. Influenza caused a deadly worldwide pandemic in 1918-1919, as populations were weakened in the aftermatch of World War I. Over 20 million died worldwide.
A worldwide H1N1 "swine flu" epidemic is underway in 2009, caused by a new strain; so far the deaths from the new strain have been few. It is highly infectious and people with the symptoms should stay home. A typical case lasts up to one week, and a person can infect others one day before they begin experiencing symptoms and up to five days after they become sick.
There is no cure--despite fake remedies sold on the internet. Once someone is already infected, Tamiflu (oseltamivir) and Relenza (zanamivir) can help relieve the severity of disease if taken within 48 hours.
The most effective protection is prevention through vaccination and the federal government plans to offer 250 million vaccine doses to the public from mid-October to the end of 2009
Flu can cause mild to severe illness, and at times can lead to death. In normal years in the U.S. on average 5% to 20% of the population catches the flu; more than 200,000 people are hospitalized from flu complications, and about 36,000 people die from flu-related causes. Victims who are over age 70, under age 5, oe with diabetes, heart disease, or asthma are at high risk for dangerous flu complications.
Telling the difference between flu and a common cold
The flu is different from a cold. The flu usually comes on suddenly and may include these "flu-like" symptoms:
- High Fever
- Extreme tiredness
- Dry cough
- Sore throat
- Runny or stuffy nose
- Muscle aches
- Stomach symptoms, such as nausea, vomiting, and diarrhea, also can occur but are more common in children than adults
Fever is rare with a cold.
Fever is usually present with the flu in up to 80% of all flu cases . A temperature of 100°F or higher for 3 to 4 days is associated with the flu.
A hacking, productive (mucus- producing) cough is often present with a cold.
A non-productive (non-mucus producing) cough is usually present with the flu (sometimes referred to as dry cough).
Stuffy nose is commonly present with a cold.
Stuffy nose is rare with the flu .
Chills are uncommon with a cold.
60% of those with the flu experience chills.
Tiredness is fairly mild with a cold.
Tiredness is moderate to severe with the flu.
Sneezing is commonly present with a cold.
Sneezing is not common with the flu.
Cold symptoms tend to develop over a few days.
The flu has a rapid onset within 3-6 hours. It hits hard and includes sudden symptoms like high fever, aches and pains.
A headache is fairly uncommon with a cold.
A headache is present in 80% of flu cases.
Sore throat is commonly present with a cold.
Sore throat is not commonly present with the flu.
The peak of flu season is winter. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. Seasons are more likely to be severe if the epidemic originates in China. The U.S. Center for Disease Control monitors circulating flu viruses and their related disease activity and provides influenza reports each week from October through May.
The period when an infected person is contagious depends on the age and health of the person. Studies show that most healthy adults may be able to infect others from 2 days prior to becoming sick and for 10 days after they first develop symptoms. Some young children and people with weakened immune systems may be contagious for longer than a week. Infectivity also varies widely between people, with about one in ten being "hyper-infectors" who are responsible for the majority of influenza transmission. While each flu season is unique, it is estimated that, on average, approximately 5% to 20% of people get the flu each year, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 people die on average per year from the complications of flu.
In 2009 the world confronted H1N1, popularly known as Swine flu, a respiratory illness caused by an influenza A virus.
In April, 2009, a worldwide outbreak of Swine Flu was reported, first in Mexico, and soon in many other countries as well, giving every indication of a possible pandemic. The Centers for Disease Control and Prevention said, The virus is a mix of human virus, bird virus from North America and pig viruses from North America, Europe and Asia. The CDC at first recommended the use of the flu drugs Tamiflu and Relenza, as there specific vaccine for H1N1 became available in late summer 2009. It is genetically different from the fully human H1N1 seasonal influenza virus that has been circulating globally for the past few years. The new flu virus contains DNA typical to avian, swine and human viruses, including elements from European and Asian swine viruses.  
Different varieties of swine flu have been known for many years. The virus routinely causes outbreaks in pigsbut seldom infect humans. Currently medical workers are exploring the possibility of a mutation of known types of the virus, enabling it to pass among species more easily. There have been cases of the virus spreading from human to human, probably in the same way as seasonal flu, through coughing and sneezing by infected people. The virus is an influenza A virus, carrying the designation H1N1. 
Spanish Flu Pandemic: 1918-1919
The Spanish Flu from 1918 - 1919 was a deadly strain of Influenza A virus which struck civilians and soldiers following World War I. This pandemic killed 50 to 100 million people worldwide, more people in one year than the Black Death, Bubonic Plague from 1347 to 1351. Indeed, more people died from the Flu than from battle injuries in the war. The flu did not originate in Spain; it was called the Spanish Flu because it received greater press attention in Spain than anywhere else in the world. More than one-fifth of the world's population was infected, in large part because of the close quarters of men living together in army camps, and because it was at the end of the war, as they returned they brought the disease back to the wider world. The dislocations and harships of war left millions of people ill-nourished and more vulnerable.
Some 675,000 Americans died. Montana was one of the four hardest-hit states in the nation, as 5,000 residents, or 1% of the population, died as a result of the infection. In response to the epidemic the State Board of Health urged closing public gathering places. Such regulation spawned public resentment, but the Board of Health stood firm. Butte was the hardest hit city of Montana and one of the hardest hit in the nation. The University of Montana in Missoula closed to protect the students. In remote areas of the state isolation and limited medical personnel left many families to face illness with help from neighbors.
This flu was most deadly for those in the healthy age groups, between 20-40, whereas most flu deaths usually occur in the elderly or the very young. The effect of the influenza epidemic was so severe that the average life span in the US was depressed by 10 years. The influenza virus had a profound virulence, with a mortality rate at 2.5% compared to the previous influenza epidemics, which were less than 0.1%. 
While the 1918 Pandemic was particularly devastating to communities and individuals, viral, host and environmental factors were at cause.
While many modern medicines such as vaccines and drugs are now available a large pandemic could still cause over 100 million deaths worldwide because of increased travel and lengthy lead times to manufacture the life saving drugs.
- Crosby, Alfred W. America's Forgotten Pandemic: The Influenza of 1918 (2003) excerpt and text search
- Iezzoni, Lynette. Influenza 1918 (The American Experience) (2000) excerpt and text search
- Jones, Esyllt W. Influenza 1918: Disease, Death, and Struggle in Winnipeg (2007), case study in Canada. excerpt and text search
- It can be spread from contact with infected birds, pigs or other animals.
- CDC: Swine Influenza
- WHO bulletin: Epidemic and Pandemic Alert and Response (EPR)
- Swine Influenza: General Information
- Pierce C. Mullen, and Michael L. Nelson, "Montanans and 'The Most Peculiar Disease': the Influenza Epidemic and Public Health, 1918-1919". Montana: the Magazine of Western History 1987 37(2): 50-61; Volney Steele, "The Flu Epidemic of 1918 on the Montana Frontier." Journal of the West 2003 42(4): 81-90
- "1918 Influenza: the Mother of All Pandemics": Jeffery Taubenberge & David Morens