From the post:
We are alerting Windows users of a new type of ransomware that exhibits worm-like behavior. This ransom leverages removable and network drives to propagate itself and affect more users. We detect this ransomware as Ransom:Win32/ZCryptor.A.
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The post goes onto note these avenues of infection:
Ransom:Win32/ZCryptor.A is distributed through the spam email infection vector. It also gets installed in your machine through other macro malware*, or fake installers (Flash Player setup).
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If you think that sounds bad, consider one of the recommended means for avoiding Ransom:Win32/ZCryptor.A:
Avoid browsing web sites that are known for being malware breeding grounds (illegal download sites, porn sites, etc.)
And the other reasons for using the Internet would be? 
BTW, the bulletin avoids the most obvious solution to Ransom:Win32/ZCryptor.A:
Don’t run Windows.
Yes?
Something to bear in mind when the GAO wants agencies to upgrade from pre-Windows software to “modern,” but insecure software.