Whenever someone is ill with a cold or flu, the 'fact' always seems to be brought up that you're contagious before you start exhibiting symptoms - coughing, sneezing etc.

Many people with flu are somewhat contagious before they begin to show symptoms. Adults with flu can be contagious as soon as 1 day before flu symptoms appear.
-- source

Is this true?

  • From a purely evolutionary angle it makes better sense for most diseases (or computer viruses) to try to be as contagious as possible before being symptomatic, or before the symptoms become too obvious/fatal. Once symptoms are obvious, defences usually come into play as you can avoid obviously infected individuals , and if the symptoms cause death too quickly then spread isn't as rapid.
    – Rory Alsop
    Mar 27 '11 at 16:35

It depends on the particular illness. With some diseases you can spread (be contagious) before symptoms appear, others you don't become contagious until after the symptoms appear.

Viruses and bacteria have evolved to reproduce and migrate in different ways. Some spread by contact and can survive in open air for a long time--these could possibly live on external skin or be on kitchen counters, desks, pens, pencils, etc. and be transported to a new host when the new host comes in contact with the infected area. These are probably also most likely to spread before a host shows symptoms.

Other types require a moist environment to survive and cannot exist in dry, open air. These would need to be spread via bodily fluids (saliva, sperm, blood, etc.) and are often spread through coughing or sneezing--hence the symptoms are there in order allow the organism to spread to a new host.

Edit (per Matthew Read's comment, here are some links):

The Wikipedia entry for Influenza describes the method of transmission for that disease. It confirms that a person with influenza is contagious before they are symptomatic.

The Wikipedia entry for Transmission (medicine) lists multiple routes of transmission for a variety of maladies, from HIV to athlete's foot.

Edit II (per Borror0's comment):

The US Center for Disease Control (CDC) offers a number of notices and other articles aimed at the general public and health care professionals. In an article on preventing the spread of the flu virus in child care settings, a brief explanation is given as to when an infected person can be considered contagious:

People with influenza can potentially infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick.

Other articles provide information on the transmission route for a variety of viruses. For instance, an article about the Norovirus says:

Noroviruses are transmitted primarily through the fecal-oral route, either by consumption of fecally contaminated food or water or by direct person-to-person spread.

Another article explains the transmission of the rabies virus:

The most common mode of rabies virus transmission is through the bite and virus-containing saliva of an infected host. Though transmission has been rarely documented via other routes such as contamination of mucous membranes (i.e., eyes, nose, mouth), aerosol transmission, and corneal and organ transplantations.

Meningitis can be caused by a bacterial infection or a viral infection. According to the CDC, viral meningitis is contagious only after symptoms appear and most stop being contagious when the symptoms go away. The same article also offers the following information about bacterial meningitis:

[S]ome forms of bacterial meningitis are contagious. The bacteria can mainly be spread from person to person through the exchange of respiratory and throat secretions. This can occur through coughing, kissing, and sneezing.

  • Please use a better source than Wikipedia to back your claims.
    – Borror0
    Mar 29 '11 at 7:37
  • 2
    @Matthew Read & @Borror0: Additional references and links have been provided.
    – oosterwal
    Mar 29 '11 at 12:46

Yes, you are infectious before you're symptomatic with the flu. This is one reason why the flu spreads so efficiently - sick people tend to stay at home, reducing the risk of infecting others.

The reason why the SARS epidemic was containable was that isolation measures could be applied to stop the spread of the disease - only symptomatic persons were infectious. If SARS had been like the flu, the world would have had a much more serious problem (anyone remember SARS at all, today)?

Here's a study about how infectiousness before symptom onset (among other things) determines whether containment strategies can work: http://www.pnas.org/content/101/16/6146.full.pdf+html

Taken from this study:

Infections before symptom onset

So, you can contain SARS or smallpox, but no chance with flu or HIV.


Do a little thought experiment. Imagine you're a virus wanting to infect as many people as possible. Your victims will also become horribly disfigured, ill, and probably die as a result of being infected.

Would you want your victims to become contagious before or after they become symptomatic?

That's how evolution works :) It's also why e.g. Ebola doesn't spread very far geographically. Victims tend to get so seriously ill so rapidly after infection they have little time to infect enough others and to travel widely. Most infections therefore will be in the immediate vicinity of the initial victim, often his or her caregivers who may get infected by accidents or carelessness while handling the victim or his remains.

HIV otoh has a very long period during which the carrier is contagious but not symptomatic, which has allowed it to spread worldwide over a relatively short period of time despite having an infection vector which makes it harder for it to actually infect another victim (direct blood contact is required, rather than merely being for example coughed on).

  • 1
    Actually, that’s not how evolution works, there is a crucial difference: your argument presupposes foresight (“want to infect as many people as possible”) which evolution lacks. There are many strategies that are in theory superior to what has evolved naturally but that evolution cannot reach because they would require foresight. In the case of infection incubation time, foresight isn’t actually necessary. Mar 28 '11 at 11:55
  • 1
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    Mar 29 '11 at 17:45
  • Please provide some references to support your claims. Mar 16 '13 at 13:01

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