What are some of the characteristics of malware that make it difficult to detect?

What will be an ideal response?

ANSWER: Some of the characteristics that make malware harder to detect and eliminate include the following:
* encryption-Some malware is encrypted to prevent detection. Most antimalware software searches files for a recognizable string of characters that identify the virus. However, encryption can thwart the anti-malware program's attempts to detect it.
* stealth-Some malware disguises itself as legitimate programs or replaces part of a legitimate program's code with destructive code.
* polymorphism-Polymorphic malware changes its characteristics (such as the arrangement of bytes, size, and internal instructions) every time it's transferred to a new system, making it harder to identify.
* time dependence-Some malware is programmed to activate on a particular date. This type of malware can remain dormant and harmless until its activation date arrives. Time-dependent malware can include logic bombs, or programs designed to start when certain conditions are met. (Logic bombs can also activate when other types of conditions, such as a specific change to a file, are met, and they are not always malicious.)