Most cases cannot be prevented. Medicines should be taken exactly as prescribed. The doses should not be more than the doctor prescribes. Avoiding heroin could prevent cases due to heroin overdose.
If the heart beats too slowly, it may not be able to pump blood well enough to keep a person alive. Most long-term effects are related to the cause. For example, cases due to hypothyroidism often go away quickly with treatment, and there are no long-term effects. Cases due to an arrhythmia or shock sometimes result in death. Head injuries and brain damage may cause permanent disabilities.
A slow heartbeat is not catching and poses no risks to others.