394 Microbes and You 



Among the smaller viruses are those of foot and mouth disease 

 (10 millimicrons), yellow fever (22 millimicrons), and polio- 

 myelitis (25 millimicrons). These are smaller than some of the 

 larger protein molecules. Among the larger viruses are included 

 those causing smallpox (250 millimicrons). For comparison, a 

 human red blood cell is 7500 millimicrons in diameter, and as many 

 as one million virus particles can be packed within a single bac- 

 terial cell. 



All viruses studied to date have been found to consist of high 

 molecular weight nucleoproteins. Viruses are resistant to cold, as 

 is indicated by certain animal viruses which can withstand — 76° C. 

 for as long as one year. Pasteurization (62° C), on the other 

 hand, will inactivate most viruses in thirty minutes or less. Lyo- 

 philization, which is freeze-drying in a vacuum, will preserve 

 viruses for indefinite periods, providing the oxygen is completely 

 excluded. Phenol, tincture of iodine, formaldehyde, and exposure 

 to ultra-violet radiation will inactivate viruses after a few minutes 

 exposure. While some antibiotics, including aureomycin, are able 

 to inactivate viruses, most antibiotics are not nearly as effective 

 against these filterable agents as they are against bacteria and 

 some of the rickettsiae. 



The high degree of specificity exhibited by viruses for certain 

 hosts— even specific tissues of specific hosts— is one of their out- 

 standing characteristics. This tissue affinity is sometimes used as 

 a basis for classifying virus diseases as: 



1. Dermotropic (associated with skin): chicken pox and smallpox. 



2. Neurotropic ( associated with central nervous system ) : rabies 

 and poliomyelitis. 



3. Pneumotropic (associated with the lungs): influenza and virus 

 pneumonia. 



4. Viscerotropic ( associated with internal organs ) : yellow fever. 



The filterability of viruses is not attributed wholly to their 

 minute size. Just as the type of filter, temperature, size of par- 

 ticles, amount of positive or negative pressure exerted on the filter, 

 pH, nature of the suspending fluid, and electrical charges are im- 



