Host cells are living cells that provide a suitable environment for viruses, bacteria, or other microorganisms to live and reproduce. These cells are essential for the life cycle of many pathogens, as they supply the necessary nutrients and machinery for replication. When a virus infects a host cell, it hijacks the cell's processes to produce more copies of itself. This can lead to cell damage or death, contributing to diseases. Understanding host cells is crucial for developing treatments and vaccines against infections caused by various pathogens, including viruses and bacteria.