The Right of the Line — Christopher G. Nuttall

The war is on the verge of being lost. 

The alien virus has begun its offensive, punching through the human defences and sending advance elements to threaten Earth itself. HMS Invincible and her crew, having barely returned from their last mission, are thrown into a desperate mission to strengthen the inner line of defences before the virus can smash them too and destroy the human race. The stakes have never been so high. Defeat means the end of everything. 

But when the virus continues its offensive, brushing through all opposition, Invincible must mount a desperate raid deep into enemy space to win time for the human race....

Or watch, helplessly, as humanity faces a fate worse than death....