(CNN) — Governments and health officials around the world continued to take steps  against the outbreak of swine flu that has killed scores of people in Mexico and spread to the U.S., Europe and possibly Asia.

Mexican health officials suspect that the swine flu outbreak has caused more than 159 deaths and roughly 2,500 illnesses.

The World Health Organization says at least 105 cases have been confirmed worldwide, including 64 in the United States; 26 in Mexico; six in Canada; three in New Zealand; and two each in Spain, the United Kingdom and Israel. WHO has confirmed deaths only in Mexico, where seven people have died from swine flu.

In the United States, California, Indiana and Texas also were reporting additional cases not confirmed by the Centers for Disease Control and Prevention.

The move means the U.N. agency has determined that the virus is capable of significant human-to-human transmission .

When the flu spreads person-to-person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight off because people have no natural immunity.

The symptoms are similar to common flu. They include fever, lethargy, lack of appetite, coughing, runny nose, sore throat, nausea, vomiting and diarrhea.

The virus spreads when an infected person coughs or sneezes around another person. People can become infected by touching something with the flu virus on it and then touching their mouth, nose or eyes.

In 1968, a “Hong Kong” flu pandemic killed about 1 million people worldwide. And in 1918, a “Spanish” flu pandemic killed as many as 100 million people. Putting those figures into perspective about 36,000 people die from flu-related symptoms each year in the United States, according to the Centers for Disease Control and Prevention.

“When you think about the flu, the seasonal flu, the flu that we’re accustomed to, it typically tends to have the worse ramifications in people that don’t have developed immune systems — the elderly and the very young. They can’t fight it off,”