Yes someone can have pneumonia and not know about it.
Some people come down with and get a milder case of pneumonia that they may think is just the flu virus because the symptoms of the mild case of pneumonia which is known as walking pneumonia may seem like the flu.
The walking pneumonia can also make you feel miserable, with cough, fever, chest pain, mild chills, headache and you might not think much of it.
Then sometimes it may get worse.
If it's not flu season and you get symptoms similar to the flu or the symptoms get worse then see a doctor or go to the emergency room.
You can also get pneumonia in one or both lungs and most times a patient doesn't realize they have it.
In more severe cases of pneumonia it can be deadly if not treated.