Malware is a term used to describe any undesired software of malicious intent. Software that falls within the generalized context of malware is categorized by its mechanism of replication and pathology (cause of infection and its effect).
A masternode keeps a real-time copy of a blockchain and is always running. Masternodes are different from a full node as it has different functions such as increasing transaction privacy, participating in a blockchains governance, voting, budgeting, and treasury system. A masternode's functionality varies between different blockchains and isn't limited to one consensus mechanism.
If a new or less mined PoW cryptocurrency uses the same hash algorithm as a more adopted cryptocurrency, a miner can merge the mining of the two without the requirement of additional computational power. Merged mining protects a new cryptocurrency from a 51% attack as its mining is more accessible to current miners. Without merged mining, a new coin is likely to have less competition between miners increasing the likelihood of a 51% attack by a single entity.