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You catch it from somebody who has the infection.
You may not know they have it because it is often very mild and sometimes causes no symptoms at all.
This is especially dangerous for young babies, who are most likely to get it from teens and adults who are unaware the have whoopingcough/pertussis infection.

The bacteria that cause it are carried in the lungs, throat and nose. So for you to catch it you have to inhale the bacteria that somebody else has coughed out. They do not live outside the body and so it has to be somebody who has coughed into the same air that you are breathing.

Although contacts in the same house are likely to get it, it can also pass easily between friends, especially children. It does not pass so easily between adults, who tend to cough away from people rather than directly over them. It is most infectious in the first 2 weeks of symptoms when it seems no different from an ordinary cough and cold. 

Many people who have whooping cough can identify the person who gave it to them. This is because it is usually somebody you have been in close contact with and because you have heard them cough the same unusual choking cough that you now have!

It is possible that it can be passed on through saliva or catarrh from an infected person. This will usually only apply to children who are unaware of exchanging such body fluids. The bacteria are soon dead outside the body.

You need to inhale very many bacteria cause whooping cough. Probably hundreds or thousands unless you are really susceptible (like the newborn).

Immunity only lasts a few years, even after the natural infection. That is why quite a lot of older children and adults get it.

There are probably other things that make people more vulnerable to catching it from time to time. In my experience I have found that having a viral cold or cough increases the likelihood of catching whooping cough. This can make the diagnosis of whooping cough even more difficult because you have two illnesses in succession. People who suffer from asthma also seem more susceptible to whooping cough, although paradoxically, asthmatics who get whooping cough often find their asthma is improved for the duration of whooping cough and for some time afterwards.

It is possible to be infectious (able to pass the infection to others) for three weeks from when symptoms first start. An appropriate antibiotic such as azithromycin kills pertussis bacteria quickly. It is considered safe to mix again after 3 days on it.