The goal of modern malware writers is to create software that is stealthy and flows undetected for as long a period of time as possible. Many anti-virus applications fail to find malware that attempts to infect systems through multiple entry points, such as e-mail or USB drives. A report found that antivirus products miss between 10 percent and 60 percent of the evasions most often used by attackers. Attackers will look for vulnerable network servers, web servers, and other areas where they can gain another foothold: weak passwords.”]
Source: https://www.csoonline.com/article/2127816/customized–stealthy-malware-growing-pervasive.html