Welcome to Android Central! Which phone do you have? I don't quite understand your question. Can all apps be downloaded where? Are you asking if all apps can be installed on the phone? Apps aren't always going to be compatible with every device--it may depend on hardware and Android version.
What apps contain a virus? If you're talking about .apk files from filesharing sites, you should avoid those completely, because they are much higher risk for malware, and they are often pirated as well (which is illegal, and should not be discussed here).
There are plenty of free antivirus apps available on Google Play. Try Lookout, TrustGo, or Avast. Google can also scan apps on your phone if you go to Settings>Security, and check "Verify Apps" on.