By Rachael Rettner, Senior Writer
Published: 02/11/2014 04:45 PM EST on LiveScience
News that NBC anchor Bob Costas will take a night off from the hosting the networks' Olympics broadcast because of an infection that spread to both his eyes might have you wondering: Why is pink eye so contagious?
Last week, Costas appeared on air with one red, swollen eye, and said he expected the eye infection to clear up by the weekend. But, in fact, the infection spread to both eyes by Monday, and Costas told the "Today" show that he would take at least one night off from the Olympic primetime broadcast.
Many things can cause pink eye, or inflammation of the transparent membrane that lines the eyelid and white part of the eye, including viruses, bacteria or allergies. The exact cause of Costas' swollen eyes has not been revealed. [5 Most Likely Real-Life Contagions]
But one of the common causes of pink eye is a virus called adenovirus, said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University in Nashville, Tenn.
This virus is highly contagious, and it's common for it to spread from one eye to the other, Schaffner said.
"We touch our eyes all the time, so you can get a little bit of virus on your fingertips," and spread it to the other eye, Schaffner said.
The virus can also spread to surfaces, such as a keyboard, and be transmitted to other individuals if they touch the contaminated surface and then touch their eyes, Schaffner said. It doesn't take much virus to cause an infection, and this contributes to its transmissibility, Schaffner said.
Although it's not clear whether Costas has adenovirus, "the fact that it goes from one eye to the other makes it a bit more likely to be classical, adenovirus pink eye," Schaffner said.
The virus does not usually cause a fever, but it can still make people feel miserable and uncomfortable, Schaffner said.
While some devoted workers may not like the idea of taking a sick day, those who have adenovirus should stay away from others until they are better, and wash their hands frequently, Schaffner said.
If Costas does have adenovirus, Schaffner said, "I would be surprised if he only had to take one night off. These infections usually interfere with your normal functioning for more than just 24 hours."