Among the various transmission routes of infection, droplets and airborne routes are the most interesting with respect to building ventilation and indoor air filtration. Examples of pathogens that can be transmitted by large droplets but are not considered as true airborne infections include SARS coronavirus (SARS CoV), whooping cough (Bordetella pertussis), influenza virus, adenovirus, rhinovirus, Mycoplasma pneumoniae, group A streptococcus, and bacterial meningitis (Neisseria meningitidis). Virtually, all pathogens where replication and/or colonization occurs in the upper respiratory tract (URT) could have the potential of being transmitted by large droplets (Chin, 2000). Siegel et al. (2004) state that only tuberculosis (mycobacterium tuberculosis), measles (rubeola virus), and chickenpox [varicella zoster virus (VZV)] could be considered as true airborne infectious diseases, although claims of such a small number of airborne transmittable diseases are debatable. For example, although a WHO consensus document concludes that SARS is mainly transmitted by large droplets, some detailed investigations concluded that some outbreaks might be airborne (Li et al., 2005; Yu et al., 2004).