Why should we wash some foods and not others?
It's good to wash fruits and veggies with cold water. Washing off the excess dirt on fruits and vegetables is important because bacteria like to hide out in the dirt. If you are washing a strong skinned fruit, such as a potato or an apple, feel free to scrub the skin with a brush to get the particles off the skin.
It is however, a BAD idea to wash meat and poultry. When we wash things, water tends to go splashing around sinks and counter tops. Therefore, by washing meat and poultry, we are splattering bacteria everywhere. It's much better just to cook meat and poultry, because the bacteria will be killed in the cooking process.
As always, don't forget to wash your hands and any surface that the meat or poultry may have touched. This will prevent cross-contamination with other foods.
Finally, do not wash eggs. Eggs (at least in USA) have been commercially washed and then coated in mineral oil which protects them. If you wash off the mineral oil, you are increasing the risk of cross-contamination especially if the eggs become cracked.