A study conducted in May 2013 by microbiologists from North Carolina State University and the University of Colorado proved an old fact: harmful bacteria can be found anywhere inside the home. The study identified more than 7,000 kinds of bacteria on things people touch, areas where food touches, and places that accumulate dust. However, a part of this figure pertains to the most dangerous and most common bacteria that can be found in any home.
Bacillus is one reason why it is a good idea to have water and air filters constantly checked and replaced especially if the home has a humidifier. If inhaled in considerable amounts, this bacteria can cause lung inflammation, sinus irritation, and hypersensitivity pneumonitis which is normally associated with the inhalation of mold spores. On the other hand, streptococcus and staphylococcus, which can be passed through skin contact, are common causes of food allergies.
E. Coli is an interesting example since while majority of E. Coli strains are at least harmless to humans, a few of them (like O157:H7) are very dangerous and can cause food poisoning. The harmful E. Coli strains are usually found in the kitchen and bathroom. E. Coli poisoning is one big reason why it is never a good idea to pick up food from the floor since it can be riddled with microscopic traces of fecal matter where E. Coli thrive.