Lawrence,
As far as taking viruses out of the air I think the only thing that would be remotely effective would be the SARS masks (or similar) like they wear or wore in China during the SARS outbreaks. The main purpose of air cleaners or purifiers is taking dust, pollens, etc., out of the air for people with asthma and allergies. Remember that viruses in particular must have a place to enter the body such as mucosal membrane areas. The main entry points are the eyes, nose and mouth. Most of us come in contact with viruses on our hands/fingers and rub our eyes. The best action is to avoid crowds and wash the hands often while avoiding rubbing the eyes. When white counts are extremely low, weigh the risks of coming in contact with others and especially question everyone who is visiting if they have had any colds, viruses, etc., or have been around anyone that has.
When the white counts are below 1500, a mask should be worn and gloves even when touching outside dirt in the garden below about 3000.
Ed