One of the most prevalent obsessions is a fear of contamination, which accounts for approximately a quarter of all obsessive themes and is the most common OCD concern worldwide. Typically the contamination worry is based on a fear of some sort of disease or illness (usually death, but sometimes other concerns such as a fear of blindness or religious concerns are a factor). For example, one might fear developing cancer or disease and so worry about toxic materials to an extreme, such as x-rays, asbestos, or many other numerous carcinogens occurring either naturally or in everyday products.

Clearly something such as asbestos is dangerous in real life (it is no longer used commonly as it is illegal, but a lot of older homes and building still have things made out of asbestos or other dangers such as lead paint), but a person with contamination OCD would take the fear to the extreme. When walking past a home with asbestos siding, the OCD sufferer might start to imagine that rain water had washed the asbestos particles onto the side walk, and now that it was dry that they might actually be stepping on asbestos particles which were then billowing into the air and attaching onto their clothes. Or even that the asbestos siding had deteriorated (even if it was clearly well encapsulated) so much that it was in the air and attaching to their clothes and indeed infecting everything in proximity. Whereas a regular person would realize that while harmful, the asbestos would really only be dangerous if they were to break it apart and start sniffing in the dust, and that while there are a lot of dangers everyday life, that if everything was so dangerous we would all be sick and dead already (in this case via cancer).

Depending on where the contamination obsession resides in an OCD sufferers' hierarchy or the severity of the OCD, determines the extremes to which these obsessions can manifest. Often times a person with contamination fears will get to the point where virtually everything except a small safe area is contaminated, since the contamination has spread. Since the obsession is in regards to disease or illness one with OCD often feels a sense of responsibility to protect others from the contaminants. This is often where tension is created or stress is caused by those close to an OCD sufferer as they will insist on friends and loved ones avoiding contaminants for fear of them being contaminated or spreading the germs to their safe zones.

While fear of illness or disease is the root of most contamination obsessions, it is not always the case. A good example are people that are bothered by sticky or greasy substances. In cases such as these often the reason for being bothered by the substance can't even be articulated except for a discomfort. Those with OCD often suffer from a heightened sense of symmetry so perhaps getting