Thomas is infected with cold (7)
I believe the answer is:
sceptic
'thomas' is the definition.
(I've seen this before)
'infected with cold' is the wordplay.
'infected' becomes 'septic' (similar in meaning).
'with' indicates putting letters inside.
'cold' becomes 'C' (eg on taps).
'septic' enclosing 'c' is 'SCEPTIC'.
'is' acts as a link.
(Other definitions for sceptic that I've seen before include "Person who doubts the truth" , "Disbeliever" , "One questioning accepted opinions" , "Person who tends to disbelieve" , "One who doubts the truth of religions" .)