Molecule loss is a critical reliability issue in diffusive molecular communications. This paper proposes a communication framework that allows biologically-enabled machines (bionanomachines) to transmit and receive information-carrying molecules (information molecules) in a robust manner against molecule losses. The proposed framework, called molecular fountain, employs deoxyribonucleic-acid (DNA) molecules as information carriers and leverages molecular fragmentation (i.e., packetization) between transmitter (Tx) and receiver (Rx) bio-nanomachines. It performs feedback-aided rateless erasure coding that considers biochemical constraints in DNA synthesis and sequencing to generate molecular packets. The Tx bio-nanomachine repeatedly generates molecular packets with Luby transform codes and transmits them to the Rx bionanomachine until it receives an acknowledgment from the Rx bio-nanomachine. The Rx bio-nanomachine can reconstruct lost molecular packets from other packets that have been successfully transmitted. Simulation results show that molecular fountain enhances robustness against molecular packet losses and in turn improves communication performance such as transmission latency, jitter, error rate, and coding overhead.