Final answer:
After Hooke discovered cells, van Leeuwenhoek discovered bacteria, one-celled protists, and sperm cells using his self-made microscopes, leading to crucial advances in microbiology.
Step-by-step explanation:
The hidden, minuscule organism that Antonie van Leeuwenhoek detected after Hooke's discovery of the cell was bacteria. Using his self-made microscopes, which were almost as powerful as some modern light microscopes, van Leeuwenhoek observed bacteria, one-celled protists, and sperm cells. These single-celled organisms were initially referred to as 'animalcules' by van Leeuwenhoek. His discoveries were fundamental to the field of microbiology and significantly advanced our understanding of the microscopic world.