Petya is a malicious software that spreads through phony attachments in emails, which when clicked upon, installs the malware on the victim’s computer and prevents access to all the files unless a ransom is paid.
1- Petya spreads by exploiting ETERNALBLUE which is a vulnerability in the Microsoft Server Message Block version 1. Therefore, to prevent the spread of Petya within the organisation, upgrade all your Microsoft OS NOW :
2- The fact that you have a Patched Microsoft OS does not prevent you from being infected by Petya if you click on an infected file in your email. Having a patched Microsoft OS simply prevents the automatic spread of it within your organisation.
3- If you don’t know/trust the sender of an email, don’t click on the darn attachment!!!