Answer: Many different viruses.
There are many different viruses that cause the stomach flu (viral gastroenteritis). Usually no one has any idea which one they have. These are the main culprits:
Rotavirus1 is the leading cause of vomiting and diarrhea in young children. Its incubation period is about 48 hours. Rotavirus lasts longer than other gastroenteritis viruses, and children are generally sicker with rotavirus than with other gastroenteritis viruses. They may have vomiting for 2-3 days and then diarrhea for an additional 4 days. The diarrhea is particularly stinky and can look greenish. Rotavirus is extremely contagious. The virus is shed in feces, vomit, and nasal secretions. Rotavirus is found in feces for about 2 weeks after symptoms have stopped so people are contagious for at least that long. My son got rotavirus when he was 22 months old and was sick for 1 full week. The diarrhea didn’t start until the 3rd day after the vomiting was over. The diarrhea was so watery it couldn’t be contained in any type of diaper. Many children (including my son) are so sick with rotavirus that they have to go to the hospital to get an IV to prevent dehydration. Hand sanitizers that are 62% ethanol do kill rotavirus after 30 seconds of contact time2. Infection is thought to produce partial immunity. That means that a child can get rotavirus again but it is usually not as bad the next time. Adults usually don’t get rotavirus but it is possible3. I did not get rotavirus when my son had it, and he vomited all over me, so I was definitely exposed. I would like to hear from any adults who did get rotavirus from their children so please e-mail me your sad story.
Caliciviruses4 are a family of viruses that include the non-enveloped Norovirus5 (Norwalk6 virus and others) and Sapovirus. All of these cause gastroenteritis and they are thought to be the main cause of viral gastroenteritis in adults. Norovirus strain GII.4 Sydney seems to be causing the most trouble in 2012-2013 but it is not the only norovirus going around. They generally have a 12-72 hour incubation period and affect children and adults. The illness typically lasts 1-3 days. Norovirus is frequently a foodborne illness. It is also extremely contagious from person-person. The viruses are present in feces and vomit. People with norovirus are contagious for at least 3 days after symptoms have stopped. These viruses withstand freezing; one outbreak of norovirus on a cruise ship was attributed to the ice cubes7. Boiling kills this virus in food. The CDC recommends using a minimum of a 2% chlorine bleach solution to kill norovirus on household surfaces. Regular 62% ethanol gel hand sanitizers do not do a very good job killing norovirus8! Yikes! Some people are less susceptible to noroviruses than others. In a study where volunteers were infected with Norwalk virus9 (anyone want to volunteer for that?) 82% became infected and 18% did not (lucky ducks). Of the 82% that became infected about 1/3 were asymptomatic and did not get sick (more lucky ducks). However, these asymptomatic people were carrying and transmitting the virus without knowing it. So, you can catch norovirus from someone who is not even sick. Having asymptomatic carriers makes containing the spread of these diseases even more difficult. In addition blood types are thought to play a role in norovirus infection10. For the Norwalk virus strain, it has been determined that people with type B blood are less susceptible and people with type O blood get the sickest11. However, for another strain of norovirus, people with type O blood were less affected12. After you are sick with norovirus, you are thought to be immune for a few months but only from that exact same strain. After a few months even that same strain can get you again. Norovirus does not play fair. Go to the CDC's web site for a wealth of information on norovirus.
Astrovirus13causes gastroenteritis primarily in children and is usually milder in adults. Its incubation period is usually 1-5 days. There are 7 types of astrovirus14. By age 5-10 years 75% of children have antibodies to at least 1 type astrovirus15. This evidence SUGGESTS (but does not prove) that infection with astrovirus MAY lead to permanent immunity from that particular strain. By “permanent”, I mean “until you are old”. There tend to be outbreaks in nursing homes, so those people have lost their immunity.
Adenovirus16 is another type of virus that causes gastroenteritis in children and adults. There are many types of adenoviruses and only 2 are known to cause gastroenteritis (Adenovirus type 40 and 41). Adenoviruses 3117 and 5218 are also suspected to cause gastroenteritis and are being investigated. The incubation period for adenovirus is about 1 week19. Adenovirus can be a tough illness because diarrhea can last 1-2 weeks20.
Enteroviruses are a large family of viruses that cause many different illnesses including polio, hand-foot-mouth disease, and respiratory illnesses . Some of these viruses also cause gastroenteritis21. They are extremely common. Many people infected with enteroviruses have no symptoms. However, some people, especially babies and children, can become very ill. Enteroviruses can cause an illness that starts with a fever and the perosn may vomit a few times. Then mild diarrhea and abdominal pain occur a day later. Then a few days later you get a cough, runny nose, and sore throat.24
Don’t let this list fool you into thinking that there are only a few different viruses that cause the stomach flu. Each of these viruses has many strains and no one knows exactly how many particular strains of gastroenteritis viruses there are. Just to give you an idea, there are 70 different strains of adenovirus type 4122. There are hundreds of different strains of norovirus23. I’m sure there are also many gastroenteritis viruses yet to be discovered, and the known viruses are always mutating and changing. So, if you were hanging onto the hope that once you “get them all” you‘ll be “done”, I’m sorry to disappoint you. Your best bet is to try to avoid a few of them. For more scientific information about all of the gastroenteritis viruses, including some I did not mention, go to this virology web site.
--Annie Pryor, Ph.D.