How do you catch it?
You catch it from somebody else who has it.
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 when it seems no different from an ordinary cough and cold.
Most 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 takes more than the inhalation of one bacterium to cause whooping cough. You probably need to inhale hundreds or thousands unless you are really susceptible (like the newborn). You also need to have no immunity to whooping cough to get it easily. Most people will be partially immune through previous infection or whooping cough immunization (shots).
Immunity doesn't last forever. 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 a maximum of six weeks from when symptoms first start. For practical purposes, most people agree that after three weeks it is most unlikely to be passed on. But the main reason for having an antibiotic is that at whatever stage the disease is at, after five (possibly three) days after starting the antibiotic, the person can be considered non-infectious and no danger to others. It is usual, however, to have an antibiotic for 3 to 7 days depending on the type.
It is well known that you can have a mild attack of pertussis (whooping cough) without it being obvious what it is, since it will not be different from an "ordinary" viral cough. Such cases must have the potential to pass it on. My own observations of whooping cough give me no reason to dispute this fact, but what I have observed is that such cases do not seem to be very infectious, since most people I have studied can identify a probable source from a clear cut case.how much protection does immunization give?