To achieve this drop dead effect you need two factors: one is insidious progression of the disease and the second is an acute shift against a vital organ.
Insidious progression can be written off as a latent, incubation period of the pathogen with an unspecified threshold. Once reached, the acute shift occurs and well inevitably death follows. As a bonus I would have the virus shed during this entire period.
That is true for some Herpes Zoster infections for example. The virus can chronically infect nerves and manifest as the Shingles rash in presence of triggering factors.
So I would suggest that your pathogen infect your target organ during the incubation/latent period to explain the targeted acute shift.
Next, the organ choice should be made. Keep in mind how flashy you want the death sequence to be.
A. I would personally go for a neurogenic shock caused by the virus destroying encephalic and spinal nerves. Some protein, a trigger would set off the bombs the virus was preparing all this time, destroying the entire neural network of the host. I can picture parts of the network being cut sequentially with the host losing control of one body part at a time every few seconds or minutes to make it more graphical.
B. Another equally graphical approach would be to target the heart. In essence, as long as there destruction of myocardium (heart muscle) some things can happen:
- Destroy a large enough part and the heart won't pump blood. The person faints, brain hypoxia follows and death.
- Destroy part of the neural network of the heart. This would have to destroy a part to create a malignant heart rate that would speed up really fast (200-300+ beats) and then either stop or keep going. The death sequence is similar to 1, aka no blood is pumped.
- Destroy many small tissue patches so that a malignant circuit is eventually formed, just like in a myocardial infarction. Make it so that a malignant rhythm develops and we're back to 1.
Another idea for this section is myocardial perforation.
C. Some other organs that would quickly lead to death if destroyed fast enough are:
- Kidneys (25% of all blood goes through them every few minutes, bleeding to death is easy)
- Lungs (hypoxia, simple and deadly or tension pneumothorax or bilateral pneumothorax, fancier and more graphical due to asphyxiation)
- Large vessels: Aorta/Vena Cava (rupture them and death is guaranteed in mere minutes)
A few considerations
From an evolutionary perspective, this seems rather improbable to occur because the pathogen could achieve absolute mortality once triggered, meaning it always kills its hosts. With no hosts, it won't be able to reproduce so its ultimate purpose is lost. No pathogen aims for death, but reproduction.
There should be cases where the pathogen is chronically infecting the host without it triggering a reaction. Also keep in mind that this pathogen would have to be able, to some extent, mimic, trick or suppress the immune system.