Influenza, commonly referred to as the “flu,” is a contagious respiratory tract infection that primarily affects the nose, throat, and lungs. It poses a significant threat to public health, leading to the loss of hundreds of thousands of lives each year.

1. Seasonal Influenza: Influenza epidemics follow a seasonal pattern. In the Northern Hemisphere, they typically occur between late autumn and early spring. During this period, two or three different strains of influenza may circulate. In contrast, equatorial regions experience influenza cases throughout the year.

2. Influenza Pandemic: Influenza pandemics can arise at any time of the year and affect populations worldwide. These pandemics occur when a new influenza A virus emerges for which there are no existing antibodies in the global population. Due to the widespread infection during a pandemic, millions of severe and fatal illnesses often result.

3. H1N1 Influenza Pandemic: The influenza A (H1N1) virus represents a novel subtype of influenza that affects humans. It results from a combination of two swine influenza viruses containing genetic elements from avian and human sources.