Question

no cells, takes over a host cell from which it replicates and spreads into neighboring host cells

Answer 1

What you are describing is the behavior of a viral infection. Viruses are microscopic infectious agents that cannot replicate independently. Instead, they invade a host cell and hijack its cellular machinery to replicate themselves. Once replicated, the viruses can be released from the infected host cell and infect neighboring cells to continue the infection cycle.