Ведь вирусу надо распространяться, а чем дольше носитель живет, тем ширее можно распространиться. Не следует ли отсюда, что в силу естественного отбора среди штаммов вируса, смертность от него постепенно должна снижаться?
Следует и наблюдается.
Чем дольше вирус живет с носителями, тем менее он для них патогенен.
Для примера:
1. Оспу вызывают два вида вирусов: Variola major (летальность 20—40 %, по некоторым данным — до 90 %) и Variola minor (летальность 1—3 %).
Оценки времени возникновения:
Цитата:
The date of the appearance of smallpox is not settled. It most likely evolved from a terrestrial African rodent virus between 68,000 and 16,000 years ago.[37] The wide range of dates is due to the different records used to calibrate the molecular clock. One clade was the variola major strains (the more clinically severe form of smallpox) which spread from Asia between 400 and 1,600 years ago. A second clade included both alastrim minor (a phenotypically mild smallpox) described from the American continents and isolates from West Africa which diverged from an ancestral strain between 1,400 and 6,300 years before present. This clade further diverged into two subclades at least 800 years ago.[38] A second estimate has placed the separation of variola from Taterapox (an Orthopox virus of some African rodents including gerbils) at 3,000 to 4,000 years ago.[39] This is consistent with archaeological and historical evidence regarding the appearance of smallpox as a human disease which suggests a relatively recent origin. If the mutation rate is assumed to be similar to that of the herpesviruses, the divergence date of variola from Taterapox has been estimated to be 50,000 years ago.[39] While this is consistent with the other published estimates, it suggests that the archaeological and historical evidence is very incomplete. Better estimates of mutation rates in these viruses are needed.
Examination of a strain that dates from c. 1650 found that this strain was basal to the other presently sequenced strains.[40] The mutation rate of this virus is well modeled by a molecular clock. Diversification of strains only occurred in the 18th and 19th centuries.
2. Корь. Летальность около 0.5%
Оценки времени возникновения:
Цитата:
Most estimates based on modern molecular biology place the emergence of measles as a human disease sometime after 500 AD[141] (the former speculation that the Antonine Plague of 165–180 AD was caused by measles is now discounted), although recent[when?] studies suggest measles potentially first appeared as early as the 4th century BCE[142][143] The first systematic description of measles, and its distinction from smallpox and chickenpox, is credited to the Persian physician Rhazes (860–932), who published The Book of Smallpox and Measles.[144] Given what is now known about the evolution of measles, Rhazes' account is remarkably timely, as recent work that examined the mutation rate of the virus indicates the measles virus emerged from rinderpest (cattle plague) as a zoonotic disease between 1100 and 1200 AD, a period that may have been preceded by limited outbreaks involving a virus not yet fully acclimated to humans.[141] This agrees with the observation that measles requires a susceptible population of >500,000 to sustain an epidemic, a situation that occurred in historic times following the growth of medieval European cities
То есть как карта ляжет. Может и за несколько тысяч лет в не смертельный не превратиться.
неживое
борьба