One of my parents, A, is going through the last months of cancer. They are forced in bed at home and constantly taking painkillers to manage the pain.

When relatives and friends come to visit, there are situations which are simply exhausting to A, to name a few:

 - Multiple visitors coming over at the same time and ending up chatting between them, increasing the noise in the room. Chatter noise, aside from being annoying on its own, forces A to talk louder to be heard, and that makes them go short on breath.
 - Visitors coming while having strong perfumes or, even worse, being sick, and carelessly attempting on greeting A. Strong scents are disturbing for A, and even getting a cold is something they cannot afford in their present condition.

Sadly it is not custom in this side of Europe to announce or plan a visit. In that case we could manage together with an agenda of the visitors. These people simply show up for visiting.

How can we manage communicating to the visitors to not be annoying/dangerous for A?

We would like to achieve that:

 - if one is sick, should simply not come. Even us getting a cold from them would be a risk for A, since we are in daily contact with A.
 - when there are too many people together, or the visit has been going on for too long, simply stop it, and have the visitors move somewhere else.