My husband has Alzheimer’s and is in a Memory Care facility. It was discovered in the ER that he had Pneumonia. He fell on two consecutive days and experienced delirium. He was taken to the ER to rule out head injury which was negative but spiked a temp while in ER. He received IV antibiotics followed by ten days of oral antibiotics. He continues with delerium and limited mobility.
You can look this up, but my understanding was that this type of pneumonia was viral, until the conditions in the lungs were ripe for the germs that need antibiotics.
He did come back to how he was before after he recovered in rehab.