The disease evoked by RNA viruses that infect the respiratory tract of people as well as animals and birds is called influenza, or the flu. The most recognizable symptoms are fever, headache, feeling of exhaustion; also a sore throat and nausea are possible as well as vomiting and diarrhea. It is normal for the illness to last approximately for one or two weeks and then recede but in some cases it can evoke serious complications and even lead to death. Other viral respiratory infections are not characterized with such outcomes.

It is normal situation when so-called seasonal flu strains appear but serious and uncontrolled outbreaks are also known. This happens when people have little resistance to the flu because of its new form; there is nothing to oppose to a new type of the flu and a lot more people get ill. Worldwide outbreaks are called pandemics and there were several such cases in the last century. The worst case of influenza was recorded in 1918. It got a name of the Spanish flu and became the reason of death of about 100 million people.

In 2009 people faced another pandemic and this time it came from Mexico. It is a new unknown before type of the fly and as people had no immunity to it was able to spread fast on an unprecedented scale. It got a name of H1N1 influenza A swine flu, or just H1N1 or swine flu, and was the first such a pandemic for so many years and caused many deaths.

Now it is important to know that influenza is caused by virus and not by a bacterium Haemophilus influenzae as it has been considered until 1933. The worst the bacteria can do is evoke lung, ear, eye, sinus, joint, and a few other infections, but not the flu.