This'll be the first of two posts - the following one will ask the same question of TB, and the contrast is actually fascinating.
But let's not prejudge the conclusion - we shall start with Neiserria meningitidis, affectionately known as meningococcus. The crucial question in determining it's infectivity is this: how does it spread?
Neiserria meningitiditis doesn't survive for long outside the human body, and nor does it have an animal vector to act as a storage vessel until it can get back into a human. Therefore it must go from one human more or less straight to the next. People are infected when they come into direct contact with the organism (usually from the nasal secretions of infected patients) or when they inhale respiratory droplets containing it. Both of these methods of contagion are vastly facilitated by viral upper respiratory infections, which frequently induce both nasal secretions and coughing.
So, what are the odds of getting it? The matter is of some importance, since Neiserria meningitidis is capable of producing a nasty meningitis (hence it's name!). Worried family members and health care workers need to know whether or not they should take antibiotic prophylaxis to try to prevent the same fate.
Here are the facts, courtesy of a fantastically interesting article from the New England Journal of Medicine, entitled "How contagious are Common Respiratory Tract Infections?" (N Engl J Med 2003;348:1256-66). Close family members of a person infected with meningococcus have anywhere between a 1:400 to a 1:50 risk of contagion, which is actually lower than most people think. Schoolmates and work colleagues are better off still - with only a 1:3000 to a 1:200 chance. And casual social contacts (those who might come into contact with the infected person infrequently for short periods of time) have no increased risk of getting the disease when compared with someone on the other side of the country.
These figures may surprise you, because there is often a good deal of panic when meningococcus is diagnosed. This is probably because of the meningitis that it causes. As these figures show, however, this risk is rather exaggerated. The author of the article states:
Despite the public fear, bordering on hysteria, that may follow a case of meningococcal disease, more than 95 percent of cases in the United States and other developed countries are sporadic. Thus, in the majority of instances, a second case does not follow a first one. This is why, when a single case occurs in a schoolchild, the Centers for Disease Control and Prevention recommends prophylactic treatment for family contacts but not schoolmates.
And health care workers? The best thing, as always, is hand washing. In the case of meningitis, unless the patient is coughing profusely (which is decidedly unlikely), it is only really upper respiratory tract secretions that are infectious. Nurses, for instance, are at a slightly increased risk of catching the disease, since they often directly exposed to secretions. On the other hand, doctors who spend only a few minutes per day by a particular patient's bedside are usually at no greater risk of infection than the population at large.