Answer: So; Colds are highly contagious so if someone you know has a cold, try not to use the same dishes, glasses, or telephone.
Explanation: The word "so" is used in so-called "so" sentences and can be used both at the beginning and in the middle of the sentence. This word is used to avoid unnecessary repetition of the verb as is the case in short answers such as "I think so" or "I said so", or as is the case here for linking sentences within a complex sentence or a like. Has the same meaning as "too", "in the same way" or "as well", and of course it is a linker that can have a coordinating, grammatical role but also conceptual if it is located at the beginning of the sentence, since then it does not have a linking role.