44 



CHAPTER IV 



VIRUSES AS ORGANISMS 



The principal reason that man is interested In viruses is that they pro- 

 duce diseases. Thus, one might say that the most important single characteristic 

 of a virus is its ability to produce disease. TOien a virus infects a host and 

 produces a disease, it invariably multiplies or is multiplied, that is, the num- 

 ber of units of virus is increased enormously. The magnitude of this increase 

 can be appreciated if we consider a few examples. It is possible to infect to- 

 bacco plants fairly regularly by rubbing onto the surface of one leaf of such a 

 plant a tobacco mosaic virus solution containing something like IQ-IO grams of 

 virus per cc . Considerably less than a tenth of a cc is spread over the surface 

 of the leaf. Thus one can say that lO-H grams of virus rubbed onto the surface 

 of a tobacco leaf can cause an infection of the tobacco plant. In a period of 

 just a few weeks, the plant will have developed a systemic virus infection, and 

 then the virus can be isolated from this plant. About one tenth of a gram of 

 virus will be obtained. Thus one can say that from iQ-H grams of virus placed 

 on the leaf, iQ-l grams of virus can be recovered. This represents a multi- 

 plication of 10 billion fold. Actually, this is a minimum multiplication. It 

 is believed that one tobacco mosaic virus rod is the only thing necessary to 

 cause the infection of the plant. Such a virus rod weighs about 10-I6 grams. 

 The total Eunount of virus recovered is, as mentioned before, io~^ grajns. This 

 would represent a multiplication of ±0^5 fold. 



When tobacco necrosis virus is rubbed onto a leaf of a tobacco plant, a 

 necrotic lesion is developed in about sixteen hours. From the size of the les- 

 ion and the concentration of virus in the tissue, one can estimate that about 

 4 X 10~7 grams of virus are contained. This represents about 10 billion virus 

 particles. Since it is believed that all of these virus particles were derived 

 from a single firus particle, it can be deduced that the virus multiplied 10 

 billion fold in a sixteen hour period. Another example can be had in influenza 

 virus. This virus has the ability to infect the embryos of chickens. Experi- 

 ments show that as little as iQ-l^ grams of influenza virus introduced into a 

 chicken embryo will initiate disease. After two days, about a milligram of virus 

 can be recovered from such an embryo. This represents an increase from io-14 

 to 10-3 grams, or an increase of about 100 billion fold. This again represents 

 a minimum estimate of influenza virus multiplication, for in all likelihood only 

 a fraction of the virus introduced actually plays a part in the initiation of 

 the virus infection. This increase in the case of influenza virus required only 

 two days, '^e can appreciate the magnitude of multiplication of this sort if we 

 ooinpare the situation with that which obtains for the human population. Evi- 

 dently, the human population developed from two individuals, originally, to 

 something like 2 billion at the present time. This represents a billion fold 

 increase. Instead of requiring a few days, this increase required many years. 

 The most conservative estimate is something like 6,000 years and other estimates 

 as great as 10 million years can be found in the scientific literature. The 

 whole point is that one cannot doubt that viruses do multiply. 



This multiplication process is, in its external aspects, similar to that 

 which one finds for living organisms. Virus multiplication might well be the 

 result of a mechanism analogous to the cell division fauniliar to all biologists. 

 In fact the 100 billion fold increase of influenza could be achieved by about 

 37 successive divisions into two. Since this process takes something less than 

 two days, the rate of division would not need to be greater than once in 80 



