The purpose of this study is to learn more about antimicrobials, especially drug levels in the CSF that would normally be used in young children as part of standard care. These drugs are approved by the Food and Drug Administration (FDA) for treating infections. They are used extensively in children in the United States, but they have not been extensively studied in children or babies. The investigators do not have very much information on how children and babies handle these drugs; that is, how long it takes for the drugs to be removed from the blood circulation and to pass out of the body and how much goes into the central nervous system. The investigators also do not have good information to guide how often the investigators should give the drugs to children and babies to get the maximum effect on infections with the minimum amount of drug. Thus, the investigators will collect more information about how these drugs should be used in children and babies by doing this drug levels research study.

Inclusion Criteria:

• Children receiving antimicrobial agents and having cerebrospinal fluid collected as part of standard of care

Exclusion Criteria:

• Lack of consent

Children (<18) receiving antimicrobial agents and having cerebrospinal fluid collected as part of standard of care