In most cases, they can still be washed together if you use enough detergent, choose an appropriate warm or hot cycle, and dry everything thoroughly. The main exception is kitchen towels that have touched raw poultry, pork, or beef, since these can carry harmful bacteria and are safer to wash separately.
Heavily greasy kitchen towels should also be separated because grease can transfer to other fabrics and make the entire load harder to clean. It is also important not to overload the washing machine, since towels need enough space for water and detergent to move through them properly.
So, kitchen and bath towels do not need separate loads every time. If they are only normally soiled, washing them together is usually fine. But towels exposed to raw meat or heavy grease are better washed separately, and simple sorting habits can help keep laundry cleaner and reduce the risk of cross-contamination.
