Why didn't any of the Exeter's crew beam down to the planet?

Actually, at least one crew member, the captain, did beam down. Maybe others did, too.

 

Maybe. Remember what happend with USS Inidanapolis at the end of WW2? Secret mission plus screwed-up paperwork. ( http://en.wikipedia.org/wiki/USS_Indianapolis_(CA-35)#Navy_failure_to_learn_of_the_sinking )

Crew basically got found by chance. What was left of them.

 

The entire landing party beamed down and then back up to the ship; Tracy stayed down on the planet surface "to arrange for a planet survey with the village elders." It was the landing party took the disease back up to the Exeter. The captain never returned to the ship after the initial beam-down.

 

But the CMO would also be the one person with the skills to avoid infection.

What sort of a timescale could we be talking about here? The time from first symptoms to total loss of mobility and cognition could be something like fifteen minutes, or two hours - but this would not be a significant factor in comparison with how fast the infection moves from victim to victim. The medical team could be monitoring the fate of a handful of patients at first, and by the time they realized the matter was out of hand (no matter whether 10 minutes or 2 hours into the process), propagation speed would decide whether they could "react" to it - say, send a distress call, organize an evacuation, whatever - because they could handle twenty patients and do the "reacting", but they could not handle 400 people all in the mid- or late phases of the infection, nor could they do any "reacting" at that point any more.

As for Tracey going nuts, it's remarkable IMHO how not nuts he was after being stranded for half a year in a village besieged by genocidal savages and blaming himself for the death of his crew. And how nuts it sounds in comparison that Kirk and Spock would refuse to consider his plea to arm the villagers to prevent genocidal slaughter.

Timo Saloniemi