The Clostridia are a class of Firmicutes, including Clostridium and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. Studies show they are not a monophyletic group, and their relationships are not entirely certain.
Currently most are placed in a single order called Clostridiales, but this is not a natural group and is likely to be redefined in the future.

C. perfringens is commonly encountered in infections as a benign component of the normal flora. In this case, its role in disease is minor. Infections due to C. perfringens show evidence of tissue necrosis, bacteremia, emphysematous cholecystitis, and gas gangrene, which is also known as clostridial myonecrosis.

The toxin involved in gas gangrene is known as α-toxin, which inserts into the plasma membrane of cells, producing gaps in the membrane which disrupt normal cellular function.
The action of C. perfringens on dead bodies is known to mortuary workers as tissue gas and can only be halted by embalming.
Some strains of C. perfringens produce toxins which cause food poisoning if ingested. In the United Kingdom and United States they are the third most common cause of food-borne illness, with poorly prepared meat and poultry the main culprits in harboring the bacterium.

The clostridial enterotoxin mediating the disease is often heat-resistant and can be detected in contaminated food and feces.
Incubation time is between 8 and 16 hours after ingestion of contaminated food. Manifestions typically include abdominal cramping and diarrhea - vomiting and fever are unusual. The whole course usually resolves within 24 hours.

Very rare, fatal cases of clostridial necrotizing enteritis have been known to involve "Type C" strains of the organism, which produce a potently ulcerative β-toxin.
It is likely that many cases of C. perfringens food poisoning remain sub clinical, as antibodies to the toxin are common amongst the population.
This has led to the conclusion that most, if not all, of the population has experienced food poisoning due to C. perfringens.

(via Wikipedia)