Condoms offer great protection against sexually transmitted infections when having vaginal or anal sex. The only problem is that certain sexually transmitted infections transmit from skin to skin contact. Since condoms do not cover all the genitals there is still a chance that these can be transmitted.
The most common of these would be the human papilloma virus (HPV).
Here is a link for more information.
http://www.sexualityandu.ca/adults/sti-1-1.aspx
The herpes simplex virus (HSV) is also another common virus.
Here is a link for more information.
http://www.sexualityandu.ca/adults/sti-1-3.aspx
Syphilis can transmit by skin to skin contact but is not as common as HPV or HSV.
Here is a link for more information.
http://www.sexualityandu.ca/adults/sti-1-8.aspx
It is difficult to work out the exact risk as it really depends on if the person has the infection in the first place.
I found one study that showed that condom usage reduced the transmission of genital HSV type 2 from infected men to women by 50%.
Most things in life carry some level of risk. For example if you drive a car there is the chance of getting hurt in an accident. You can reduce your risk by driving a safe car, sticking to speed limits, not drinking alcohol, wearing seatbelts etc.
With sex there will always be some level of risk both physical and emotional. Using condoms and both people testing will really reduce this risk.
The good thing with sex is that you can have different types of safer sex such as hand jobs, penis between ladies breast, erotic massage.
HN