The virus operates by replacing the standard Menu Definition with its own virus code. This operation will first be performed on the System file, and from there it will spread to other applications.
On advanced Macintoshes (the IIci and the
IIfx), the standard Menu Definition is in ROM and its use is not overridden by the virus. This prevents the virus from spreading from the System file to applications.
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It has been shown that on newer Macs running recent System software (eg. the Mac IIci running System 6.0.5) the virus cannot spread at all.