From January  2010 issue of Safety+Health:

“CDC and OSHA estimate as much as 40 percent of a workforce could be affected by a pandemic and be absent from work due to illness, having to care for a sick family member, or having to care for children in the event of a school closure.  In addition to absenteeism, organizations must contend with presenteeism–when a sick worker is physically present.  According to CDC, an individual who comes to work with the H1N1 virus will infect 10 percent of the workforce.”