Its very possible that they would carry something that could harm the other... but it isn't a general rule that they DO.. just that they can... hence why there should always be a quarentine period with any animal... The reason you were probably told that is because they come from different areas of the world.. therefore one may have a parasite or bacterium the other has no immunity to.. Not to mention, boa's get signifigantly bigger than Ball's... so they shouldn't be kept together anyway...its not wise to keep any herp's of different species in the same enclosure.. unless they all come from one part of the world.. and even then it gets a bit complicated.
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Lucien
1.0 Columbian Redtail Boa (BCI)
3.1 Leopard geckos (2 Blizzard and 2 het Blizzard)
0.1 Savannah Monitor
13 rats
12 Gerbils
2 Dogs
3 cats
1 Albino Corey (fish)