Difference between common cold and flu?

Question

What are the differences between flus and colds? I don't know about English, but in Spanish they confuse it very often when using words gripe and resfriado.

Maybe it's not the case in English, but I would like to know what are their differences and if you can give a bunch of examples for each one similar to this way: you know that you have flu/cold when...

In addition, both flu, cold and allergies in the nose (e.g. with allergy: dust) have significant symptoms related with the nose. It is many times with mucus, sneezing or stuffy nose.

How can you recognize if it's allergy, flu or cold? I didn't want to take the general case of allergy because not always it is related to the nose.

Answer 1

Symptoms, in short:

Common cold: blocked nose, yellow/green mucus, scratchy throat lasting for 7-14 days (usually no headache, fever or fatigue), year round

Hay fever (allergy to pollens): runny nose (not really blocked), clear mucus, itchy eyes, lasting for several weeks, mainly in spring (usually no headache, fever or fatigue)

Flu (seasonal influenza): Headache, high fever, muscle pain and profound fatigue that needs bed rest. Fever usually last for less than a week, mainly in winter (October - March).

Even shorter: common cold = thick, yellow mucus; pollen allergy = thin, clear mucus; flu = headache + fever + fatigue

Sometimes, common cold can present with mild fever and mild headache in which case you may not tell if it is cold or mild flu.

Source: Cold vs Flu Ehealthstar.com

Answer 2

In general a cold (usually rhinovirus), Influenza, or even a bacterial infection can have quite similar symptoms. If contracted through airborne particles, they primarily affect the respiratory system. Mucus, sneezing, fever, are all signs that the body is fighting an infection, as is fatigue as the body diverts resources to the immune system. Essentially, especially at a mild level it is difficult to tell the difference between these without a lab test. When it becomes serious - high fever, extreme fatigue - is it unlikely to be a cold. Any time you have these sort of symptoms you should seek medical attention, because it could be any number of other flus or viruses, some potentially life threatening, especially in small children, the elderly, or those already weakened by another condition.

Allergies are a whole different thing, with a slightly different immune response, and it generally depends on how it manifests. Hayfever might produce mucus or breathing difficulty due to inflammation or muscle spasm in the airways, and can be life threatening in rare cases, even without pre-existing asthma, but it is unlikely to produce fever.

I'm unsure how to reference this, as it's generally covered in a first year biology text, but the Mayo clinic is perhaps the most reliable and independent source for general medical questions.

http://www.mayoclinic.org/diseases-conditions/common-cold/expert-answers/common-cold/faq-20057857

http://www.mayoclinic.org/diseases-conditions/flu/basics/definition/con-20035101