The two principal processes by which viruses penetrate host cells are (engulfment) and direct fusion.

The two main processes by which viruses penetrate host cells are endocytosis (engulfment) and direct fusion.

1. Endocytosis (Engulfment): This is a process where the virus is engulfed or 'eaten' by the host cell. The virus attaches itself to the host cell's surface and the cell membrane wraps around the virus, forming a vesicle. The virus is then brought into the cell within this vesicle.

2. Direct Fusion: In this process, the viral envelope directly fuses with the host cell membrane, allowing the viral genome to enter the host cell. This usually happens when the virus and host cell have proteins on their surfaces that can bind to each other, triggering the fusion process.

Both of these processes allow the virus to inject its genetic material into the host cell, which then uses the host's machinery to replicate and produce more viruses.