ideally....
A delicate task. It will be hard for the offending person to hear, so handle it with diplomacy and sincerity. Tell the individual that you would like to speak with him or her. Set up a meeting or a lunch - strictly private. There must be no chance of another coworker walking in on this conversation, so set aside a special time and place.
Tell your coworker you have noticed lately that there is a problem. If it's a body odor problem, tell him or her you have noticed lately that their deodorant is no longer working. Suggest that every so often, deodorants quit doing their job and that a solution could be as simple as changing brands.
Be non-accusatory; be positive. Your role is that of a supporter and a helper.
Tell them you'll keep checking back to see if the problem is solved. I often hear from folks that say the talk did some good for awhile but, for whatever reason, the coworker went back to how it was before. So, they need to know you are expecting change, and that you are going to follow up.
False sincerity shows through. If you feel that you cannot be sincere, elect another of your coworkers to take on the task. Also, it will probably be more comfortable for the coworker with the problem to hear this information from someone of the same sex.
Hibe