Is drinking from a single vessel by multiple individuals consistent with the practice of the Companions, or does the spread of diseases via saliva make this act undesirable?

The default rule is that it is permissible for more than one person to drink from a single vessel, and for them to share food from one vessel, as the Prophet (peace be upon him) and his Companions did. However, in cases of illness or fear of contagion, a Muslim should not drink from the leftover drink of a sick person, because prevention is commanded. Related to this is the prohibition of blowing into food and drink, and his (peace be upon him) saying: "A sick person should not be brought to a healthy one."