You catch it from somebody who has the infection.

 You may not know they have it because they are in the early stages. Or because sometimes it is quite mild and sometimes even no symptoms at all.

The worse it is the more infectious it is, so mild cases may not pass it on so easily. Those without symptoms possibly don’t pass it on at all, but nobody really knows.

 Whooping cough  is especially dangerous for young babies, who are most likely to get it from an unimmunized sibling, or a teen (often a sibling), or an adult (often a parent), who is unaware they have whooping cough.

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. The bacteria do not live outside the body for long and so it usually has to be somebody who has coughed into the same air that you are breathing. It may also pass through mucus transfer that occurs with kissing or though coming into contact with an infected person’s mucus and finger-licking etcetera, such as young children frequently do.

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. 

About half of sufferers know who they caught whooping cough from.

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!

Whooping cough is possibly passed on via nasal discharge or saliva.

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 to whooping cough doesn’t last.

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.

Asthma sufferers are more susceptible to whooping cough. 

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 4 weeks from when symptoms first start. In some cases it might be even longer. An appropriate antibiotic such as azithromycin kills pertussis bacteria quickly. It is considered safe to mix again after 3 days on it. See antibiotics in whooping cough page.