APPENDIX P 
The scenarios produce the following formulae; 
First pathway of infection: 
P, • Pj N Pj X P, X P5 X P, - C, (death by infection) 
P, X Pj X X X p^ X P, X p, X P, 
Cl (resistant carrier, 
causing others' 
death) 
P, > P, P, X P, X Pi X P. X P, X P, X P, 
— Cj (epidemic) 
Second pathway of infection: 
P, X P, X Pj X P, X Pj X P,j = C, (death by cancer) 
P, Pi P, X P, X Pi X P„ V p„ 
- C, (cancer epidemic) 
To what extent can actual values be given to Pi — Pn? 
I realise I run the risk of ridicule in attempting to assign 
such values in the following section. Nevertheless I think 
it is important to do so, and for other scientists to think 
about the probabilities they themselves would assign. 
Most microbiologists working with non-pathogenic 
bacteria or other micro-organisms have at some time or 
other swallowed some viable cells, but I would be surprised 
if they did so more often than about once in 100 experi- 
ments, so the value of Pi will be low, perhaps ~ 0-01. 
For estimates of P; and Pj, I will rely on experiments 
carried out with volunteers by Professor E. S. Anderson, 
one of our experts in the field of plasmid transfer. He 
showed that E.coli K12 can survive in the gut for a few 
days and that a plasmid could be transferred to resident 
baderia at very low frequency. The transferred plasmid 
did not spread; instead it soon disappeared. In these ex- 
periments with plasmids the volunteers ingested lO'* bac- 
teria; Anderson concludes that the possibility of plasmid 
transfer would be i emote if, under more realistic circum- 
stances, someone swallowed just a few thousand bacteria 
{Nature, vol 255, p 502). Let us again be pessimistic and 
assume P; and Pi together equal 01. 
On the other hand, we have already seen that the 
probability of a plasmid containing a latent virus or 
oncogene must be low (P, = 0 0001) simply because the 
mammalian genome is so large and fragments in the 
plasmid DNA are so small. I have already mentioned that 
no mammalian virus has ever been grown in a prokaryotic 
cell. Therefore, P.i is likely to be extremely low; neverthe- 
less I will assign a value of say 0 001. P«, P,, P» and P. are 
very difficult to assess. If the virus is pathogenic, it is likely 
to be transmissible, but extremely dangerous organisms 
such as the (Ireen Monkey virus causing Marburg disease 
are fairly easily recognised and the people infected readily 
isolated in hospitals. I would describe P«, P, and Ps as un- 
likely, but P; as quite possible. I will however assign the 
fairly high probability of 0 • 1 to all four of these events. 
Something can be said about Pn, since in spite of 
intensive efforts by many laboratories so far it has not 
been shown that any human cancer is caused by a virus. 
It is much more likely that natural and man-made chemicals 
in the environment are responsible for most human cancer 
(see, for instance. The Cancer Problem by John Cairns 
Saenttfic American, vol 233, p 64). Both events P,„ and 
Pii are therefore very unlikely but I again take the pessi- 
mistic view and give them values of 0- 01. We can now tot 
up the probabilities of the various deleterious consequences; 
Cl 10 * X 10 ' X 10 ‘ X 10 * y. 10 ‘ - 10 " 
C, ■ 10 " X 10 ' X 10 ' = 10 ■> 
Cj = 10 ’• X 10 ' 10 ‘4 
C, = 10 '• X 10 « X 10 * 10 '< 
With the exception of Pi and Ps, I believe I have assigned 
generally high values to all the other events which must 
take place before any biologically hazardous situation 
arises. Nevertheless, the cumulative values above repre- 
sent extraordinarily small probabilities. Thus, if 10 scien- 
tists in each of a 100 laboratories carried out 100 experi- 
ments per year, the least serious accident (Ci) would occur 
an average once in a million years. 
It could be argued that many of the probabilities have 
been misjudged. For instance, the lowest, Ps, could be said 
to be misleading because in screening for the required 
clone, many hundreds of thousands of bacterial colonies 
have to be picked and sub-cultured, and therefore the 
chance of one of these having a latent virus is very much 
higher. Although large numbers of clones may be screened, 
almost all the important prolonged experiments (the 100 
experiments I referred to above) will be carried out with 
one or a few types of cloned plasmid. The essential point 
is that even if the product of the probabilities is under- 
estimated by several orders of magnitude, the chance of 
an accident from studies in heterogenetics remains 
vanishingly small. Nevertheless, it can never be excluded. 
What I find extremely disquieting, is the degree of 
concern which has been generated in a world where far 
greater hazards should be everyone’s concern. How can 
scientists accept a world bristling with nuclear weaponry, 
with the capacity of killing several times over every 
member of the human race, while at the same time demand- 
ing super-safety measures for shotgun or related experi- 
ments in heterogenetics? 
Apperudix P — 2 
