The World Health Organization raised its flu alert level to 6 Thursday, declaring the first flu pandemic in 41 years.

So what does that mean?

Flu pandemics occur when a strain of the flu virus mutates into a new form that can spread from human to human and to which people have no natural immunity.

Because there is no widespread immunity to the new strain of H1N1, its effects are worse than the normal flu. It is also difficult to predict which age groups will be worst hit by a pandemic strain.

Why is swine flu a bigger concern than seasonal flu, which sickens hundreds of thousands and kills between 30,000 and 40,000 people in the U.S. each year?

Scientists say it’s because swine flu is killing not only those in “high risk” groups, i.e. the elderly, the very young and those with compromised immune systems, it’s also killing healthy people.

There were three pandemics in the 20th Century: in 1918 (Spanish influenza); 1957 (Asian influenza); and 1968 (Hong Kong influenza). According to the WHO, the 1918 pandemic killed between 40 million and 50 million people worldwide, making it one of the deadliest pandemics in human history.

What Does Level 6 Mean?

Raising the alert to level 6 means that swine flu has reached widespread human infection.

This means governments should ready their pandemic preparedness plans and increase detection systems for potential cases, which most are already doing.

This also confirms that swine flu is spreading in a second region outside the Americas, although it's described as "moderate in severity."

Many countries had asked the WHO to hold off on such a declaration, fearing their citizens will be alarmed and confused, and that governments will be pressured to institute costly and often ineffective measures like trade and travel bans.

But with the most recent statistics showing that swine flu has spread to 74 countries, sickening more than 26,500 people and killing at least 140 — although health officials say the infection and death rate is likely much higher — the WHO has opted to raise the alert to its highest level.

Still, the WHO says that a pandemic declaration does not mean the situation is worsening, and countries do not need to take new actions. Aphaluck Bhatiasevi, a communications officer for the WHO, told FOXNews.com that countries should continue to follow the recommended guidelines for level 5.

The current actions of the United States and Mexico, the two hardest-hit nations, likely won't change. Some other governments may step up their efforts to contain the virus. Vaccine makers are being urged to switch to swine flu vaccine once they finish making seasonal flu vaccine.

Click here to learn more about flu alert levels at the WHO’s Web site.

The Associated Press contributed to this story.