One who touches parts of the body which are usually covered, such as one’s shoulders or feet, one must wash one’s hands. One may also not recite words of holiness, such as blessings or Torah study, until one does so.
Hagaon Rabbeinu Yosef Haim zt”l writes in his Responsa Torah Lishma (Chapter 14) that if one touches a body part that is usually covered, one must wash the entire hand and merely washing that part of the hand or fingers that touched the body part is insufficient. For instance, if one touched one’s foot or leg with one’s finger, washing that finger is not sufficient and one must wash one’s entire hand. The Mishnah Berura (Chapter 613, Subsection 6) and Yalkut Yosef (Chapter 4, page 399) rule likewise, although there are those that disagree.
Netilat Yadayim in this context refers to washing one’s hand under the faucet and one need not necessarily use a vessel. If one touched the sole of one’s foot, one should preferably wash one’s hands three times (see Responsa Yabia Omer, Volume 5, Chapter 1, Section 5).
The Poskim disagree regarding one who touches a baby whose body is not usually as covered as adults. For instance, during the summer months when it is commonplace for a baby’s legs and feet to be uncovered, if one touches a baby’s feet, since for the baby, these areas of his body are not truly considered “covered”, must one wash one’s hands or not?
Clearly, this refers to touching a baby’s feet when they are clean, for if they are soiled or if one touches the soles of the baby’s feet, one must certainly wash one’s hands.
Halachically speaking, Hagaon Chazon Ish ruled stringently in this regard as is quoted in his name in several books (see Piskei Teshuvot, Volume 1, Chapter 4, note 228). Nevertheless, Hagaon Harav Ben Zion Abba Shaul zt”l is quoted as having ruled leniently (see Ohr Le’Zion, Volume 2, Chapter 44, Section 6). This seems to be the prevalent custom nowadays. We have heard Maran zt”l rule likewise.