Your baby wants to be with you. You are the person your baby knows and loves more than all others. He knows when you are not with him. This makes him fear that will not return. This distress is called "separation anxiety". Separation anxiety is an important part of growth.
Help your baby learn to separate from you. Try not to leave when your child is hungry, tired or sick. Ask the person who will care to stay with your baby and show him an interesting toy. Let the baby get used to the new person. Give your baby a comfort object, like a bear or a soft blanket. This can help bring comfort while you are away.
Although the baby cries loudly, you should not go in hiding. Tell your baby what will happen while you are away. Tell your child when you will return. Say for example, "I need to go now. Aunt Mary is going to play with you and feed you. I'll be back before you sleep. I love you". Give baby a kiss and leave. They will soon cease to mourn, and learn that you always come back over time.