There is malware in the wild that can affect Macs, but it can all be avoided by practicing safe computing, as described in the link I posted earlier. No, 3rd party antivirus software is not required to keep your Mac safe from malware.
You don't need "cleaner" or "maintenance" apps to keep your Mac running well, and some of these apps can do more harm than good. Some can even degrade, rather than improve system performance.
Some remove files/folders or unused languages or architectures, which does nothing more than free up some drive space, with the risk of deleting something important in the process. These apps will not make your Mac run faster or more efficiently, since having stuff stored on a drive does not impact performance, unless you're running out of drive space.
Some of these apps delete caches, which can hurt performance, rather than help it, since more system resources are used and performance suffers while each cache is being rebuilt. Caches exist to improve performance, so deleting them isn't advisable in most cases.
Many of the tasks performed by these apps should only be done selectively to troubleshoot specific problems, not en masse as routine maintenance.
Mac OS X does a good job of taking care of itself, without the need for 3rd party software. Among other things, it has its own maintenance scripts that run silently in the background on a daily, weekly and monthly basis, without user intervention.
Five Mac maintenance myths