Histopathology image classification plays a critical role in clinical diagnosis. However, due to the absence of clinical interpretability, most existing image-level classifiers remain impractical. To acquire the essential interpretability, lesion-level diagnosis is also provided, relying on detailed lesion-level annotations. Although the multiple-instance learning (MIL)-based approach can identify lesions by only utilizing image-level annotations, it requires overly strict prior information and has limited accuracy in lesion-level tasks. Here, we present a novel severity-guided multiple instance curriculum learning (Su-MICL) strategy to avoid tedious labeling. The proposed Su-MICL is under a MIL framework with a neglected prior: disease severity to define the learning difficulty of training images. Based on the difficulty degree, a curriculum is developed to train a model utilizing images from easy to hard. The experimental results for two histopathology image datasets demonstrate that Su-MICL achieves comparable performance to the state-of-the-art weakly supervised methods for image-level classification, and its performance for identifying lesions is closest to the supervised learning method. Without tedious lesion labeling, the Su-MICL approach can provide an interpretable diagnosis, as well as an effective insight to aid histopathology image diagnosis.