Causes, Risk Factors and Prevention of Cold and Flu

By Nancy Baxi, M.D. ; Updated August 14, 2017


Both colds and the flu are caused by viruses. Examples of viruses that cause the common cold include rhinovirus, adenovirus, coronavirus, respiratory syncytial virus (RSV), influenza, parainfluenza, enterovirus and metapneumovirus. Parainfluenza and RSV tend to cause more colds in children, while rhinovirus is the most common cause overall. These viruses are further divided into more than 200 subtypes, therefore, even if you catch one virus and develop an immunity to that particular one, there are still many other viruses that can give you a cold.

There are three types of influenza virus: A, B and C. Types A and B are responsible for the clinical syndrome of the flu. Type C only causes mild cold-like symptoms. Influenza A has many subtypes, some of which can infect animals and can be responsible for pandemics like the swine flu. Influenza viruses mutate very easily as well, making a different flu virus to fight off every year.

Transmission and Prevention

Cold and flu viruses reside within droplets that float in the air or rest on objects, such as doorknobs. When someone is coughing or sneezing they release these droplets (aka aerosolized secretions) into the air. If another person breathes in these droplets, or touches the doorknob while the virus is still alive, they may become infected.

Luckily, viruses don't survive within the droplets for too long. Some cold viruses can live on surfaces, such as toys or doorknobs, for as long as one day. People with colds typically carry the cold virus on their hands, where it can infect another person for at least two hours. Therefore, it is very important to wash your hands many times throughout the day to physically wash off any viruses that you pick up from hand-shakes, doorknobs or other objects in your surroundings.

Influenza can live on the hand for two hours and on objects between two and eight hours. It will incubate in the body for 18 to 72 hours before symptoms begin. This is dependent on how large the virus load introduced into the body is (i.e., a large, close-proximity sneeze has a much higher virus load than a doorknob). People are more infectious earlier rather than later in their course of illness, such as the first two to four days.

Higher Risk

Stress, lack of sleep, smoking and poor nutrition can increase the odds of catching the cold or flu because these factors decrease one's immune system. Heavy physical training may also increase risk, while moderate physical activity may keep you healthy. Patients with congenital immunodeficiency disorders and those with weakened immune systems (i.e., from chemotherapy, immunosuppressant drugs, chronic steroid use and leukemia) are at higher risk for catching colds and the flu and typically have more severe symptoms.

Flu Vaccine

The flu vaccine may help prevent the flu and its complications. The flu vaccine is a small amount of inactivated flu virus introduced into the body. The immune system creates antibodies against the flu in response to the vaccine, so when the person is exposed to the virus in real life, they already have antibodies to fight it off. The main concern, however, is that the flu virus mutates, so the flu vaccine is only effective if it addresses the strain of flu virus the person is exposed to. Every year a new vaccine is developed based on scientists’ best guesses as to what the strain of flu will be in the upcoming year. The flu vaccine is available as a shot or an intranasal spray (such as FluMist).

The flu vaccine is recommended for people between the ages of 2 and 49. The vaccine has many restrictions, for example it is contraindicated for those who have an egg allergy, a history of Guillain-Barré syndrome, are pregnant, have asthma or are immunocompromised. FluMist is a live vaccine, therefore it has more restrictions. Both the shot and the spray take roughly two weeks for the person to develop immunity.

Since the elderly cannot mount a good immune response to the vaccine, it is helpful for others that live in the house or others who come into contact with the elderly person to get the flu vaccine to minimize the elderly patient’s possibility of catching the flu.

The side effects of the flu vaccine are local soreness (from the shot), generalized body aches and a low-grade fever for a couple of days as the body mounts an immune response. For nasal FluMist, the side effects last for a day or two and are runny nose, sore throat, chills and headache.


The antivirals that are used for flu treatment can also be used for prevention for those who are at high risk of getting the flu. Oseltamivir (Tamiflu) is recommended by the CDC and American Academy of Pediatrics (AAP) for the prevention of influenza in persons age 3 months and older. Zanamivir (Relenza) is recommended for the prevention of influenza in persons age 5 years or older. It is common to develop a resistance to antivirals (similar to antibiotics), therefore it is important to talk to your physician before taking them.

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