I've used Linux as my sole desktop computer operating system for 11 years and I'm currently an android developer, so I thought I'd chime in here with my feeling on the matter.
Android is Linux, and Linux is more secure than some other operating systems. Granted there is no such thing as a secure OS, but there are no active Linux viruses and there haven't been for a while. Read
this article to understand why Linux is safer. After reading that article you'll understand why it's a waste of time to write a virus for Linux and why it won't matter how many people use Linux the virus count won't grow much.
Also, each process in Android runs in its own sandbox. This setup keeps one process from affecting other processes unless two or more process are specifically written to interact.
Do you need anti-virus software in Linux? That's your decision, but many Linux users will tell you it's a waste of time. What we really need to worry about is social engineering.