The phone is not jailbroken I assume. There is no way of getting a virus on your iPhone. However I would try the following.
Close the Mail App completely from the multitasking window by double clicking the Home button and swiping up the Mail preview window all the way up.
Then do a hard reboot. Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears. Ignore the "Slide to power off" text if it comes up. You will not loose anything.
Then go to the Mail account under Settings, delete the account and add it again.