Here's why:

* Attachment and Penetration: The phage initially attaches to the host cell and injects its DNA. This stage is reversible; the phage can detach if conditions are unfavorable.

* DNA Injection: Once the phage DNA enters the host cell, the phage coat remains outside. The DNA is now in control.

* Replication and Assembly: The injected phage DNA takes over the host cell's machinery, using it to produce new phage proteins and replicate its own DNA. This process is irreversible.

* Lysis: The host cell eventually lyses (bursts), releasing hundreds of new phage particles to infect other bacteria.

Therefore, the key moment of irreversible infection occurs when the phage DNA enters the host cell. After this point, the phage has established its control over the host cell's machinery and the process of phage replication is inevitable.