Influenza virus circulating in the community as a pathogen since 16th century is known for its unique ability to cause recurrent epidemics & global pandemics. Genetic re-assortments in the virus cause fast and unpredictable changes in the virus antigenic structure causing the recurrent epidemics of febrile illnesses. Each century has seen some pandemics of such novel viruses to which the overall population holds no immunity causing huge morbidity and mortality.

The 2009 flu pandemic is a global outbreak of a new such strain of H1N1 influenza virus, often referred to as swine flu ". It is officially called pandemic H1N1/09 virus by the WHO while the CDC refers to it as "novel influenza A (H1N1)". The virus, first detected in April 2009, contains a combination of genes from swine, avian (bird), and human influenza viruses.

Till December, 2009, worldwide more than 208 countries have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including at least 10582 deaths. The actual numbers of HINI cases would be much higher.

The symptoms of swine flu are similar to other influenzas, and may include a fever, cough (typically a "dry cough"), headache, muscle or joint pain, sore throat, chills, fatigue, and runny nose. Diarrhea, vomiting, and neurological problems have also been reported in some cases. People at higher risk of serious complications include those aged over 65, children younger than 5, children with neurodevelopment conditions, pregnant women (especially during the third trimester), and those of any age with underlying medical conditions, such as asthma, diabetes, obesity, heart disease, or a weakened immune system (e.g., taking immunosuppressive medications or infected with HIV).

The World Health Organization reports that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza. Many severe cases have occurred in previously healthy people. In severe cases, patients generally begin to deteriorate around 3 to 5 days after symptom onset with rapid deterioration and progression to respiratory failure within 24 hours.

Confirmed diagnosis of pandemic H1N1/09 flu requires testing of a nasopharyngeal, nasal, or oropharyngeal tissue swab from the patient. Real-time RT-PCR is the recommended test as others are unable to differentiate between pandemic H1N1/09 and regular seasonal flu. However, most people with flu symptoms do not need a test for pandemic H1N1/09 flu specifically, because the test results usually do not affect the recommended course of treatment. The CDC recommends testing only for people who are hospitalized with suspected flu, pregnant women, and people with weakened immune systems.

Spread of the H1N1 virus is thought to occur in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing or sneezing by people with influenza. Sometimes people may become infected by touching something – such as a surface or object – with flu viruses on it and then touching their mouth or nose. The basic reproduction number (the average number of other individuals that each infected individual will infect, in a population that has no immunity to the disease) for the 2009 novel H1N1 is estimated to be 1.75

It is suggested that personal measures to avoid seasonal flu infection are applicable to the 2009 pandemic as well: vaccination when available, thorough and frequent hand-washing, respiratory etiquette, a balanced diet with fresh fruits and vegetables, whole grains, and lean protein, sufficient sleep, regular exercise, and avoiding crowds. Smoking raises the risk of contracting influenza, as well as producing more severe disease symptoms

As of November 19, 2009 (2009 -11-19), over 65 million doses of vaccine had been administered in over 16 countries; the vaccine seems safe and effective, producing a strong immune response that should protect against infection

Overall the safety profile of the new H1N1 vaccine is similar to that of the seasonal flu vaccine, and fewer than a dozen cases of Guillain-Barre syndrome have been reported post-vaccination. Only a few of these are suspected to be actually related to the H1N1 vaccination, and only temporary illness has been observed

People in at-risk groups should be treated with antivirals (oseltamivir or zanamivir) as soon as possible when they first experience flu symptoms. The at risk groups includes pregnant women, children under 2 years old, and people with underlying conditions such as respiratory problems. People who are not from the at-risk group who have persistent or rapidly worsening symptoms should also be treated with antivirals. These symptoms include difficulty breathing and a high fever that lasts beyond 3 days. People who have developed pneumonia should be given both antivirals and antibiotics, as in many severe cases of H1N1-caused illness, bacterial infection develops. Antivirals are most useful if given within 48 hours of the start of symptoms and may improve outcomes in hospitalized patients. In those beyond 48 hours who are moderately or severely ill antiviral may still be beneficial.