<s>
Oligomorphic	B-Application
code	I-Application
,	O
also	O
known	O
as	O
semi-polymorphic	O
code	O
,	O
is	O
a	O
method	O
used	O
by	O
a	O
computer	O
virus	O
to	O
obfuscate	O
its	O
decryptor	O
by	O
generating	O
different	O
versions	O
of	O
it	O
,	O
in	O
order	O
to	O
evade	O
detection	O
by	O
antivirus	O
software	O
.	O
</s>
<s>
Oligomorphic	B-Application
code	I-Application
works	O
by	O
randomly	O
selecting	O
each	O
piece	O
of	O
the	O
decryptor	O
from	O
several	O
predefined	O
alternatives	O
.	O
</s>
<s>
At	O
run	B-Library
time	I-Library
,	O
these	O
components	O
can	O
be	O
combined	O
in	O
various	O
ways	O
to	O
create	O
new	O
,	O
distinct	O
versions	O
of	O
the	O
decryptor	O
.	O
</s>
<s>
The	O
first	O
known	O
virus	O
using	O
oligomorphic	B-Application
code	I-Application
was	O
the	O
Whale	B-Application
DOS	I-Application
virus	I-Application
,	O
identified	O
in	O
1990	O
,	O
which	O
chose	O
from	O
a	O
few	O
dozen	O
distinct	O
decryptors	O
.	O
</s>
<s>
The	O
first	O
Windows	B-Application
95	I-Application
virus	O
using	O
oligomorphic	B-Application
code	I-Application
was	O
the	O
Memorial	O
virus	O
,	O
which	O
could	O
generate	O
96	O
distinct	O
decryptor	O
patterns	O
.	O
</s>
