Annual flu 
Another name for seasonal flu.

Antibiotics 
Antibiotics are medicines designed to kill bacteria and to treat and prevent bacterial diseases and infections. Antibiotics are not used to prevent or treat influenza (which is a virus, not a bacteria) but may be used to treat bacterial infections, such as pneumonia, that may occur as complications of influenza infection.

Antibody 
An immunoglobulin, a specialized immune protein, produced because of the introduction of an antigen into the body, and which possesses the remarkable ability to combine with the very antigen that triggered its production.

Antiviral 
An agent that kills a virus or that suppresses its ability to replicate and, hence, inhibits its capability to multiply and reproduce.

Avian (or bird) flu 
Flu caused by influenza viruses that occur naturally among wild birds. The H5N1 variant is deadly to domestic fowl and can be transmitted from birds to humans. There is no human immunity and no vaccine is available.

Bird flu 
Another name for avian flu.

Bronchitis 
Inflammation of the mucous membrane of the bronchial tubes. It can be caused by viral or bacterial infections or by allergic reactions to irritants such as tobacco smoke. The disease is characterized by low-grade fever, chest pains, hoarseness, and productive cough. Acute bronchitis is rarely serious in otherwise healthy adults, but it can be dangerous in infants, children, or adults who suffer from underlying respiratory disease, especially emphysema.

Common cold 
A viral upper respiratory tract infection. This contagious illness can be caused by many different types of viruses, and the body can never build up resistance to all of them. For this reason, colds are a frequent and recurring problem.

Epidemic 
A disease outbreak in which some or many people in a community or region become infected with the same disease.

Fever 
Although a fever technically is any body temperature above the normal of 98.6 degrees F. (37 degrees C.), in practice a person is usually not considered to have a significant fever until the temperature is above 100.4 degrees F (38 degrees C.).

Flu 
Short for influenza.

Influenza 
Influenza is caused by viruses that infect the respiratory tract which are divided into three types, designated A, B, and C. Most people who get the flu recover completely in 1 to 2 weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. 

Mutation 
A permanent change, a structural alteration, in the DNA or RNA.

Pandemic flu 
Virulent human flu that causes a global outbreak of serious illness. Because there is little natural immunity, the disease can spread easily from person to person.

Pandemic 
An epidemic that spreads throughout the world.

Pneumonia 
Inflammation of one or both lungs.  Pneumonia is frequently but not always due to infection. The infection may be bacterial, viral, fungal or parasitic.

Seasonal (or common) flu 
A respiratory illness that can be transmitted person to person. Most people have some immunity, and a vaccine is available.

Stomach flu 
So-called "stomach flu" actually has nothing to do with the influenza (flu) virus. This term is sometimes used to describe gastrointestinal illnesses caused by other microorganisms.

Vaccines 
Microbial preparations of killed or modified microorganisms that can stimulate an immune response in the body to prevent future infection with similar microorganisms.

Virulent 
Extremely noxious, damaging, deleterious, disease-causing. 

Virus 
A microorganism smaller than bacteria, which cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity or with errors (mutations)-this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment more difficult.