Pregnant women are 20 times as likely as other healthy young women to contract listeriosis, probably because in pregnancy the immune system is dampened to prevent rejection of the fetus.
lessen in force or effect; "soften a shock"; "break a fall" check; keep in check (a fire) make vague or obscure or make (an image) less visible; "muffle the message" reduce the amplitude (of oscillations or waves)
To dampen something such as someone's enthusiasm or excitement means to make it less lively or intense. Nothing seems to dampen his perpetual enthusiasm To dampen something down means the same as to dampen it. Although unemployment rose last month, this is unlikely to dampen down wage demands The economy overheated and the Government used to interest rates to dampen it down