In article <Pine.SUN.3.91.950928091456.10830A-100000 at alsys1>, Peter Charles <pcharles at aecom.yu.edu> says:
>Immunity to any one of the 4 serotypes (by previous exposure)
>does not confer protection against the other 3, and is in fact thought to
>exacerbate the disease upon exposure to one of the other serotypes (a
>phenomenon known as "immune enhancement" of disease). This is thought to
>be one of the mechanisms by which simple dengue fever becomes dengue
>hemorrhagic fever/shock syndrome (DHF/SS). To my knowledge, there are no
>strains (ie, D1-D4) that specifically *cause* DHF/SS.
Several "risk factors" for DHF/DSS have been identified. Around 90% (?) of
haemorrhagic fever cases occur during secondary infections (hence the
immune enhancement theories), and a secondary infection with dengue 2 is
more likely to cause haemorrhagic fever than a secondary dengue 1, 3 or 4
infection. A number of other factors such as age, sex and immune status
have also been implicated.