Viruses compete by being as small and as adaptable as possible. They are not very complex: rather than carry around the baggage necessary for arcane tasks like respiration, metabolism, and locomotion, they only have enough DNA or RNA to get themselves replicated. For example, any particular influenza strain is many times smaller than the cells it infects, yet it successfully mutates into a new strain about every other flu season. Occasionally, the virulence goes way up, and the resulting epidemic kills a few million people whose immune systems are not nimble enough to kill the invader before it kills them. Most of the time they are nothing more than a minor annoyance—unavoidable, yet ubiquitous.
The features of a good virus are:
Unix possesses all the hallmarks of a highly successful virus. In its original incarnation, it was very small and had few features. Minimality of design was paramount. Because it lacked features that would make it a real operating system (such as memory mapped files, high-speed input/output, a robust file system, record, file, and device locking, rational interprocess communication, et cetera, ad nauseam), it was portable. A more functional operating system would have been less portable. Unix feeds off the energy of its host; without a system administrator baby-sitting Unix, it regularly panics, dumps core, and halts. Unix frequently mutates: kludges and fixes to make one version behave won't work on another version. If Andromeda Strain had been software, it would have been Unix.
→ Unix is a computer virus with a user interface.