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Bird flu is caused by the influenza A type of virus which is hosted by birds and some animal species like pigs, horses, etc. and just like any other influenza virus it propagates via air, water, direct contact, body fluids and excreta. With the flu virus being airborne, the contamination in air could be reasonably high during epidemics and/or pandemic outbreak. Infected people carry the flu virus with them wherever they travel and spread it in the air and on objects they touch via their nasal secretions and bodily discharges. This makes such diseases like human influenza and bird flu contagious.
The preventive measures include combinations of various partly effective methods like vaccination, immunization, isolation, mass killing (when poultry birds get infected), and use of Personal Protective Equipment (PPE).
One such PPE device is a mask worn by people living or working in contaminated environments. Nose and mouth respirators or 'Masks' contain filters which keep out microscopic viruses of sizes as low as 0.3 microns. Typically the droplets containing the flu virus in a human sneeze, cough or spit are 0.5 micron wide and are difficult to filter using conventional (surgical) masks.
Usage of procedural and surgical masks has been widespread for curtailing the spread of common respiratory diseases. Recently they were used by personnel involved in controlling SARS. However, Centre for Disease Control and Prevention (CDC) reports that there have been no scientific studies which unquestionably establish masks as 100% effective in preventing the spread of the flu virus.
The masks that are recommended for especially bird flu are N-95, NIOSH approved ones which are capable of filtering out 95% of 0.3 micron wide flu viruses. WHO recommends strictly using these masks when in the vicinity (within 3 feet) of a potentially infected person. Also the masks should be disposed off, after one single use. Another set of very powerful, protective masks are Powered Air-Purifying Respirators (PAPRs). These are used by persons who cannot wear N-95 masks, like people with facial hair.
There is more to mask usage than just filtering out viruses. Masks can also help in many indirect ways apart from blocking flu viruses directly, however we must bear in mind an unfortunate fact that no mask is a guarantee against protecting its user from the viruses. It's just that they dramatically cut down the probability of infection. There is no single way of preventing bird flu until a generic vaccine is developed. Until then we'll have to rely on a combination of various measures to keep ourselves safe.
Clive Jenkins recommends the Bird Flu Helpsite which will provide you with comprehensive information on all aspects of the Bird Flu Virus. To find articles, tips and free advice check out the site here: http://www.birdfluhelpsite.com