  We theoretically investigate the unconventional superconductivity in the
newly discovered infinite-layer nickelates Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ based on
a two-band model. By analyzing the transport experiments, we propose that the
doped holes dominantly enter the Ni $d_{xy}$ or/and $d_{3z^{2}-r^{2}}$ orbitals
as charged carriers, and form a conducting band. Via the onsite Hund coupling,
the doped holes are coupled to the Ni localized holes in the $d_{x^{2}-y^{2}}$
orbital band. We demonstrate that this two-band model could be further reduced
to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi
liquid state above the critical $T_{c}$ could stem from the carriers coupled to
the spin fluctuations of the localized holes. In the superconducting phase, the
short-range spin fluctuations mediate the carriers into Cooper pairs and
establish $d_{x^{2}-y^{2}}$-wave superconductivity. We further predict that the
doped holes ferromagnetically coupled with the local magnetic moments remain
itinerant even at very low temperature, and thus the pseudogap hardly emerges
in nickelates. Our work provides a new superconductivity mechanism for strongly
correlated multi-orbital systems and paves a distinct way to exploring new
superconductors in transition or rare-earth metal oxides.
