GIDEON is a straightforward solution for the day-to-day treatment of Infectious Disease. It has a larger number of individual drugs, side effects, drug interactions, and trade names as well as an option to list drugs that are effective for any combination of pathogens. This is particularly useful in treating people who return from Africa, Asia, often infected by three or more intestinal parasites.
GIDEON also has good historical coverage on drugs that were used decades ago but are no longer licensed.
MIMS Online and Lexicomp have more detail on the use of drugs in specific situations. These sites offer pages of text for the management of AIDS in complex situations, treatment of tuberculosis that is resistant to standard therapy, long-term management of leprosy, etc.
There is also extensive material regarding mechanisms of drug resistance - a subject that is very involved and time-consuming to follow, requiring a specialist team.