Im not 100 percent sure. I suppose it could if the virus was written specifically for the operating system. But a virus made for PCs probably wouldn't have an affect on your phone. However, a downloaded file that is infected with a virus for PCs can probably be transferred to your computer when you plug it into it.
Definitely. Android is a particularly juicy target, because of its open nature and general lack of knowledge from users that there is actually a risk on their "cell phones". I believe the goal of most Android malware is to steal private personal information for use in identity theft.
There are free antivirus apps out there if you want malware protection on your phone.