So?
How about the FIRST link in the search:
http://www.howtogeek.com/141944/htg-explains-why-windows-has-the-most-viruses/
Why Windows Has the MOST Viruses
The Sad Security History of Windows
Historically, Windows was not designed for security. While Linux and Apple’s Mac OS X (based on Unix) were built from the ground-up to be multi-user operating systems that allowed users to log in with limited user accounts, the original versions of Windows never were.
DOS was a single-user operating system, and the initial versions of Windows were built on top of DOS. Windows 3.1, 95, 98, and Me may have looked like advanced operating systems at the time, but they were actually running on top of the single-user DOS. DOS didn’t have proper user accounts, file permissions, or other security restrictions.
Windows NT – the core of Windows 2000, XP, Vista, 7, and now 8 – is a modern, multi-user operating system that supports all the essential security settings, including the ability to restrict user account permissions. However, Microsoft never really designed consumer versions of Windows for security until Windows XP SP2. Windows XP supported multiple user accounts with limited privileges, but most people just logged into their Windows XP systems as the Administrator user. Much software wouldn’t work if you did use a limited user account, anyway. Windows XP shipped without a firewall enabled and network services were exposed directly to the Internet, which made it an easy target for worms. At one point, the SANS Internet Storm Center estimated an unpatched Windows XP system would be infected within four minutes of connecting it directly to the Internet, due to worms like Blaster.
In addition, Windows XP’s autorun feature automatically ran applications on media devices connected to the computer. This allowed Sony to install a rootkit on Windows systems by adding it to their audio CDs, and savvy criminals began leaving infected USB drives lying around near companies they wanted to compromise. If an employee picked up the USB drive and plugged it into a company computer, it would infect the computer. And, because most users logged in as Administrator users, the malware would run with administrative privileges and have complete access to the computer.
etc.
"Windows is a big target because it powers the vast majority of the world’s desktop computers and laptops. If you’re writing malware and you want to infect average computers users – perhaps you want to install a key logger on their systems and steal their credit card numbers and other financial data – you would target Windows because that’s where the most users are.
This is the most common argument for Windows having such a history of malware, and it’s true"
Maybe both you and your customers might be better off if you just didn't work on Windows systems any more.