Oscar,
It is certainly true that for diseases which have been “designed” to be transmitted by breath, such as flu, there is a period where the victim is infective but asymptomatic. This almost has to be for the disease organism to propagate. However, plague was “designed” to be transmitted via vectors (rats and fleas). It invades the lungs almost by chance. It doesn’t *need* to be transmitted by breath. Therefore there is no need to keep the victim ignorant of his infective status for any length of time. In fact, pneumonic plague is rapidly debilitating and lethal. This is certainly true for the normal progression: bubonic to septicemic to pneumonic. Such a victim is bedridden at the septicemic stage (which can last for as little as 24 hours before death. Such a victim is a danger only to caregivers (direct, close contact). For the others, who get pneumonic plague directly from another pneumonic victim, there is certainly a period of time while the bacteria are multiplying in the lungs before the new victim gets sick. At first there are not enough bacteria to cause a cough. Then there are. This quickly becomes a racking cough – not good. At this stage the victim is VERY dangerous. Then the victim becomes too sick to circulate and becomes dangerous only to caregivers. The disease acquired in this way can take as long as 2-4 days to kill. However, the period between the first cough and incapacitation is very short. Also, to be susceptible, the general population has to be unaware of the danger. How long can the general population remain ignorant? It is treatable in the very early stage (say within 24 hours of exposure) and simple precautions, like wearing a surgical mask, are effective prevention. A population at risk can avoid an epidemic by wearing such a mask, taking antibiotics at the first sign of lung distress, quarantining victims, taking a week vacation and remaining indoors, and so forth. In this, plague is very similar to inhalation anthrax, it kills too quickly to be a serious epidemic risk (so long as people know what to do). It can be a major one-time mass killer, but so were a couple of Boeing jetliners.
