Highly pathogenic or low pathogenic
Pathogenic means the ability to cause disease. A flu virus can be highly pathogenic or it can have low pathogenicity. A highly pathogenic flu virus can cause severe illness and death. A low pathogenic flu virus is much less likely to cause severe disease.

H5N1
The avian flu outbreak currently of concern is a highly pathogenic influenza of type A and subtype H5N1.  The H5 and the N1 designations refer to the types of the hemagglutinin (H) and neuraminidase (N) proteins found on the surface of the virus itself.  There are 16 different H and 9 different possible N proteins known to exist in nature. For technical information about how influenza viruses are classified, see www.cdc.gov/flu/avian/gen-info/flu-viruses.htm

Influenza (or flu)
The illness caused by influenza (flu) viruses.  Symptoms in people include fever, exhaustion, severe aches and pains, and inflammation of the mucus membranes of the throat and lungs.  Some people are at high risk for medical complications of influenza.

Influenza viruses:  One of three influenza viruses, Influenza virus A, B, or C.  Influenza A viruses are found in many different animals including ducks, chickens, pigs, whales, horses, and seals.  Influenza B viruses circulate widely only among humans.  Influenza C virus causes mild respiratory illness and is not thought to cause epidemics.  Influenza types A or B viruses cause seasonal epidemics of disease almost every winter. In the U.S. , these winter influenza epidemics can cause illness in 10% to 20% of people and are associated with an average of 36,000 deaths and 114,000 hospitalizations per year. Each year, the vaccine for seasonal flu protects against two types of influenza A and one type of influenza B virus.

Pandemic: A disease outbreak in humans that occurs all over the world.

Pandemic flu is a new human flu virus that has changed significantly from previous human flu viruses.   Because it is so different, people have little natural protection (immunity) and the disease can spread easily from person to person and cause a global outbreak, or pandemic, of serious illness. Currently, there is no pandemic flu outbreak occurring.

Seasonal (or common) flu: a respiratory illness that can be transmitted person to person.  Three types of influenza virus typically cause seasonal flu in people.  Because the flu virus changes from year to year, vaccination is recommended each year with a vaccine designed to protect against each of the three types of flu expected to cause human illness that year.