Which type of virus is able to alter its own code to avoid being detected by anti-virus software?

Macro viruses embed themselves in document macros and execute within applications like Microsoft Office; they do not alter their own code to evade detection.

A polymorphic virus uses a mutation engine to rewrite its own code - changing variable names, instruction order, or encryption keys - with each replication cycle, producing new binary signatures that bypass static signature-based antivirus detection while preserving the original malicious functionality.

A logic bomb is malicious code that triggers upon a specific condition or date; it does not self-modify its code to avoid antivirus detection.

Boot sector viruses infect the master boot record or volume boot record of storage media; they rely on execution at boot time rather than code mutation to persist.