Translation:Do not put the finger in the mouth because it is full of bacteria!
The usage of "că" to introduce causal subordinates is standard (you find it listed in dictionaries, such as DEX), unlike that of "cause" in English. My understanding is that it is (often) perceived as somewhat less stressed than "pentru că" and more commonly found in oral speech than in written texts. There are contexts in which using "pentru că" might perhaps sound unnatural / give away that you are not a native speaker.
The Romanian sentence uses "plin", which is masculine and, thus, unambiguously refers to "degetul". Although its most immediate English translation is "full", "plin" can also be referred to surfaces to mean that they are "completely covered with something" or figuratively to mean that "there is plenty of something" (in this context, both meanings apply).