Viruses cause the common cold and flu, two contagious respiratory infections. The length of time during which a person can pass on the disease varies.

Many people with flu or the common cold do not visit a doctor, and the Centers for Disease Control (CDC) do not consider adult deaths from flu to be nationally notifiable. Tracking how many people have either illness can be difficult.

However, both are common and contagious. Knowing when a person who has flu or cold is contagious can help prevent the spread of respiratory illness.

In this article, we look at both conditions and when they are most contagious.

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A cold spreads while symptoms, such as sneezing, are still showing.

People spread the common cold most often through sneezing, coughing, and touching contaminated surfaces.

This suggests that people are most contagious once symptoms become obvious, although people might be contagious before symptoms begin to show.

The response of the immune system to the virus causes the symptoms of the common cold, rather than the virus itself.

As the symptoms of the cold begin to improve, the risk of spreading the illness also reduces. However, the risk of being contagious is still present. The average cold lasts up to 10 days, so people should expect to be contagious during that period.

The seasonal cold is one of the most common respiratory sicknesses in humans.

According to the CDC, millions of people get the common cold each year in the United States.

A large number of different viral strains cause the symptoms of a cold, such as rhinoviruses, adenoviruses, and coronaviruses.

These viruses also evolve constantly to adapt to their environment and survive the human immune system. This is why no cure is currently available for the common cold.

Flu symptoms also generally last up to 10 days, depending on the individual.

A person can expect to be able to spread the flu one day before their symptoms begin and then for up to seven days after becoming ill.

The influenza virus, or flu, is a highly contagious respiratory virus that can infect people of all age groups. The severity of an influenza infection can depend on the person.

Symptoms can range from mild in some people to life-threatening in certain at-risk groups, such as older adults.

Viruses can spread through a process called viral shedding. When a virus finishes its replication cycle within a cell, it releases new viruses. These new viruses can then infect more cells and the body can expel them to transmit to other people.

These viruses exit the body in a number of ways, such as mixing with the mucus and saliva. These substances then find their way back into the open environment through things such as coughing and sneezing.

One study in The Pediatric Infectious Disease Journal found that children have a longer period of viral shedding before the onset of symptoms when compared to adults.

The study also found children aged 0–5 years had the longest period of viral shedding after symptoms had resolved.

Symptoms are the main difference between the flu and a cold. Flu symptoms are more severe than the effects of a cold. They also affect the whole body. In comparison, the common cold typically only causes symptoms in the upper airway.

Like other viruses, flu viruses are constantly evolving and changing. Therefore, the body rarely deals with the same flu virus twice. This can lead to some people contracting the flu almost every year.

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Thorough hand hygiene can help limit the spread of cold and flu.

The most important factor in reducing the spread of viral infections is creating healthful habits during cold and flu season.

Drinking plenty of water, eating a healthful diet, and getting plenty of rest are advised. These simple practices can help reduce symptoms and make any time with the illness as comfortable as possible.

When symptoms are showing, a few steps can help protect others from catching the illnesses, including:

  • Covering a cough: When people cough or sneeze, cover the mouth and nose with a tissue or the elbow of a sleeve if no tissue is available. The spray of saliva or mucus contains viruses that may increase the risk of spreading.
  • Getting rid of tissues: If possible, discard tissues immediately after use. Using a new tissue each time decreases contact with the virus and keeps hands as clean as possible.
  • Washing frequently: Reducing the spread of the cold and flu virus can be as simple as regularly washing the hands, especially after coming into contact with bodily fluids. If possible, people with cold or flu should try to avoid direct contact with other people or public items like telephones and door handles.

If symptoms become difficult to manage, some people choose to stay home from work or school and opt out of social activities. This helps prevent the spread of illness while also taking care of personal well-being.

Symptoms of the common cold

Once they infect the body, viruses spread rapidly. The symptoms of the common cold develop as the immune system responds to eliminate the common cold virus.

While the effects of a common cold are usually mild and resolve without treatment, they can be irritating and disruptive.

Symptoms of the common cold include:

  • a sore throat
  • congestion in the nose and sinuses
  • a runny nose
  • sneezing
  • hoarseness of voice
  • a cough

For a small number of people, the common cold also increases the risk of other illnesses entering the body. These illnesses include ear infections and more serious respiratory illnesses, such as pneumonia.

The symptoms of flu are similar to those of the cold but are typically more intense in comparison. Symptoms of the influenza virus typically include:

  • fever
  • chills
  • runny nose
  • cough
  • sore throat
  • muscle or body aches and pains
  • headaches
  • fatigue, which may linger after other symptoms have passed

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A range of medicines are available over the counter to treat the symptoms of cold and flu.

Both cold and flu cause symptoms that can disrupt daily life. In most cases, viral infections run their course relatively quickly without any treatment.

Many people find relief after treating the symptoms of these illnesses, however. A wide range of drugs help treat the symptoms of cold and flu. These are available over the counter (OTC).

There is also a selection of cold and flu medication available for purchase online.

These medications may be useful if severe symptoms make normal function difficult. In some people with influenza, doctors may prescribe antiviral medications.

Some people choose to take preventive action against flu in the form of vaccines. Creating a vaccine for a constantly evolving virus is difficult.

However, the CDC report that the vaccine reduces the risk of contracting the flu by 40–60 percent if the vaccine is suitable for the particular type of virus that is circulating.

Many other factors can restrict the effectiveness of a flu vaccine, but the CDC advises that the vaccine is the best way to prevent the flu.

Some people believe that the flu vaccine can cause the flu. However, it cannot. After the injection, some people experience aches in the arm the next day. This is a mild immune reaction to the shot and should resolve in a day or two.

Because the flu shot takes two weeks to become fully effective, some people might also have already caught the flu when they get the shot or shortly afterward.

Manufacturers produce the flu vaccine either from inactive viruses or viral components, as opposed to a "live" virus. Therefore, people cannot get the flu from the flu shot.

In most cases, treating the symptoms of the virus and taking steps towards healthful lifestyle choices can help prevent the spread of illness.