Yes there is a chance of disease. But, the cows do look thin. If the drought is as bad as you say, a really thin cow like that is going for $.35 to maybe $.40 lb. here. Prices have fallen off in the last month. If they weighed 900 lbs. then they are worth about $300 -$350. So I don't think you stole them but you got a good deal. I would definitely WORM them, give them some good feed or pasture, and then see.
There is a chance they are "short bred" and the blood test has a window of under 25 days and a smaller window of 35-40 days where there is some room for mistake. We do milk preg check on a few dairies, and they suggest a recheck on anything less than 40 days. It seems to be real accurate at 29 to 35 days, then again after 45 days. Ultra sound is accurate after 28-30 days, BUT, have seen cows come back in heat if they are less than 35 days bred. They tend to slip it more the shorter bred that they are. Same with palpation. I don't like to preg check beef animals before 35 days minimum and prefer 40 and over. I want that egg well implanted before it has any challenges.
We have had the vet miss some short breds and put those cows back for the next calving group, only to have them up and have a calf when we weren't expecting it. We try to have a defined calving season of 90 days, but have 2 places that we cannot get the bulls out until we preg check and wean calves. So most have been in the 5 month range, but there might be an occasional one he calls open or short bred. They often will go with the next calving bunch. Sometimes they will drop a calf, but often they get bred right back and calve early in that later group. They get one "freebie" like that unless they are a 1st calf heifer that raised a nice calf. Then she gets a pass. But anyone else, if they have any breeding problems after that, they get on the cull list.