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More than 200 different viruses are known to cause colds, with rhinovirus being the most common source. Between 30 and 35 percent of all adult colds are caused by rhinoviruses, according to the National Institutes of Health (NIH). Science has identified more than 110 of these types of cold viruses, which grow best at about 91 degrees Fahrenheit (32.8 degrees Celsius), the approximate temperature inside the human nose.

Coronaviruses also are known to trigger many adult colds, especially during the winter and early spring. There are more than 30 types of coronaviruses, but just three or four affect humans.

More severe viral sources of colds include the parainfluenza virus and respiratory syncytial virus. While these viruses do not usually cause significant problems for adults, they can cause severe lower respiratory infections in young children.

About 10 to 15 percent of adult colds are caused by viruses that typically are responsible for more severe illnesses, according to the NIH. These include:

• Adenoviruses
• Coxsackie viruses
• Echoviruses
• Orthomyxoviruses

According to the NIH, the exact viral source of 30 to 50 percent of adult colds remain unidentified.

Researchers are investigating the mechanisms behind the viruses that cause colds for a variety of reasons. They are working to understand how cold viruses infect the body and better ways to prevent or treat colds. Some scientists are even looking into using modified versions of certain cold viruses to help fight cancer without harming normal tissue.