How do influenza pandemics occur?
Human influenza pandemics can occur when a “new” type of influenza virus emerges that has the following properties: • Humans must have little or no immunity to the virus • The virus can cause significant illness or death • The virus can be spread easily from person to person. If these three criteria are met, the virus can then spread globally – this is known as a “pandemic.” The severity of the pandemic depends on the qualities of the virus as some cause more severe illness and are more infectious than others. Will A/H5N1 cause the next pandemic? Influenza A/H5N1 has met the first two of the criteria above (humans have no immunity and it causes significant illness) but, at this time, it is unable to spread easily from person to person. If A/H5N1 develops the ability to spread easily from person to person, it could cause a pandemic of similar severity to the 1918-19 “Spanish Flu.” It is unknown if, or when, this might occur. Conclusions Influenza viruses are unpredictable. We cannot tel
