Speaking from a medical point of view, it is VERY unlikely you will catch a disease froom a toilet, unless, of course, you drink its water. If someone in the household has an infectious disease, everyone should wash their hands thoroughly after using the toilet, and the ill person should have a separate water glass. If the person has an infectious SKIN disease (NOT likely-) plans should be made to wipe the toilet seat upper and lower after this person uses it. Use an all purpose anti-bacterial cleaner for this.
I have found that for the average homeowner, they should make it a habit to clean the bathroom every single week, and it helps to schedule it on a particular day. I chose Sundays to do this, as its a quiet day in my home. And this is a thorough,, deep cleaning, not just a wipe down.