Are viruses contagious through toilets?

Yes, any physical: Or airborne droplet contact may transmit viruses. Note this has been happening since the day you were born so most viral transmissions do not cause harm. Use common sense cleanliness - avoid direct skin contact on public toilet seats (or wipe with bleach towelette), wash hands thoroughly with soap and water, leave the place clean for the next user. At home use diluted bleach.

Low risk...: ...and easy to avoid. The main infections one might acquire in public toilets are colds, flu, and various gastrointestinal infections like viral gastroenteritis ("stomach flu"). There is little or no risk from toilet seats themselves, and for sure no risk of intimate infections like STDs and HIV. You can eliminate all risk by washing hands with soap and water after using the toilet.