This study aimed to examine the overnight temperature pattern of babie
s during the prodromal phase of minor illnesses. The overnight rectal
temperature pattern of 123 babies was recorded weekly from about 6 to
at least 16 weeks old, while parents maintained detailed records of si
gns of illness. By analysis of patterns of signs and visits to the gen
eral practitioner, 86 periods of minor illness were identified, mostly
upper respiratory tract infections, though it was not usually possibl
e to identify the infection by conventional virology. Data were analys
ed separately for babies who had developed an adult-like night time te
mperature pattern and those who had not. In both groups, obvious signs
of illness were preceded by a disturbance of night time temperature p
attern. Temperature was significantly raised over control weeks, thoug
h few babies were clinically febrile. The greatest temperature disturb
ances were seen in the three days before illness, though some disturba
nces were seen up to seven days before. A similar disturbance of tempe
rature was seen the night after diphtheria, pertussis, and tetanus imm
unisation, and individual responses to natural infection and immunisat
ion were well correlated, suggesting that the temperature change is mo
re a function of the host response than the infecting agent.