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Viruses spread easily in PCs because most PCs lack internal controls and many PC users share software. Because most PCs are designed to be single user systems in which the operating system and the user share the same address space, they do not have the internal controls that normally protect z/OS facilities, such as the Authorized Program Facility (APF), supervisor state, storage protection keys, access control software, and data set passwords. All executing programs have access to everything in the computer. So, nothing stops a malicious program from attacking either the operating system or application programs. In addition, the large‑scale sharing of programs among PC users contributes to the rapid spread of contaminated software.
In theory, it might be possible for PCs to contribute to the spread of viruses if they emulate 3270‑type terminals. Potentially, a virus could infect the PC, detect 3270 emulation, and insert viral source code during your TSO session. In this scenario, the infected PC would intercept and momentarily suppress the resulting TSO dialog until after the mutant form of the z/OS code was successfully implanted. The infected PC might even purge all spooled job information by entering appropriate SDSF commands or job control language (JCL) statements. You probably would not question the delay, assuming it was caused by a brief peak in the z/OS work load.
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