A Macro is a short passage of code attached to Microsoft Office Word Processing documents. They are written in the programming language Visual Basic, and are set to trigger on launch on Microsoft Office files or upon pressing a certain button or key. They can also be exploited to execute malicious code on an unsuspecting victim. These are not bots, but Macros are scripted hacks. These can be turned into viruses easily.
These scripts can't do that much other than turn itself into a virus. When a macro turns into a virus, it can easily turn a small fraction of script into a dangerous virus that easily spreads. To spread easily, it repeats the script quickly in interminable multitudes. Whenever they turn into viruses, the scripts will become more lengthy and harmful. Scripts like these end up slowing down the user's computer. Stormworm is a good example of being an invisible virus or trojan. Many viruses are made out of Macros.
Never underestimate a Macro.