A very useful peice of scientific equipment but also an EPIC way to bamboozle friends and even enemys.
To call someone a petri dish is like calling sombody a douche bad or an idiot.
A place that is full of people with the possibility of infection of viruses or diseases via close contact via people or "gross" environments, humid and warm temperatures, and people who are not minding etiquette in spread of coughing, sneezing, washing hands, or touching food.
John, that party sucked last night! Everybody was sticking their hands in the food and being all sweaty around each other. What a petri dish.