What do 即 (jí) and 已 (yǐ) do in the sentence 病征在成年早期即已显露 ("symptoms become visible at early adulthood")?
病征在成年早期即已显露


即 here means 就. It means this sense: 



So, 病征在成年早期即已显露 is equal to say 病征在成年早期就已显露.