Recently, there have been many papers published about computer simulations and mathematical models related to different fields of science and engineering. Most of these mathematical models contain ordinary or partial differential equations. In some cases, instead of integer order, fractional order can be used to analyse the real phenomena behind the problems. In one way or another, researchers encounter many kinds of nonlinear differential equations. As it is clearly known, there is no general analytical formula to solve most of the nonlinear problems and for this reason, plenty of published mathematical papers are devoted to solving or at least to approximate nonlinear mathematical models. The main ideas in these papers are to get novel approaches for complex cases of problems. If an exact solution does not exist, then one can use the approximate results obtained from the novel approaches. By doing so, one can get some knowledge about the phenomena behind the problems.

Undoubtfully, one of the most crucial problems for human beings is infectious or non-infectious diseases such as cancer, HIV, AIDS, etc. Today, human beings are under the threat of coronavirus and still try to find a cure. Understanding the transmission characteristics of infectious diseases in living species can help for decreasing the transmission of these diseases. To investigate the reasons and remedies of these diseases, many scientists have constructed certain models for them. While modelling according to situations or surrounding media, one can make assumptions and some precautions can be taken. For instance, investigations about cancer have been progressively increased as it remains one of the leading causes of global morbidity and mortality. As a colossal health anguish worldwide, cancer touches nearly every place and it is still growing at an alarming rate. It is predicted that 2 million new cases and 14 million deaths will take place in 2035 alone. To handle this brutal disease efficaciously, a joint achievement by both healthcare communities and scientists is required to capitulate significant progress in cancer research and healing methods. With the latest inventions on biotechnology, it is the necessity to establish innovative computational techniques for thorough data analysis to recuperate our understanding of diseases. Hence, new and original statistical or computational methods are needed to find the solutions that will be interpreted for investigating the behaviour of the problems.

The aim of this Special Issue is to collate original research articles that focus on the recent results - which can be obtained from the novel methods constructed by many researchers - for all types of these diseases, as well as developments in recent methods with new operators or approximations. We also welcome review articles discussing the current state of the art.

Potential topics include but are not limited to the following:

• Analytical and numerical techniques for virus models such as HIV, Ebola, and COVID-19
• Mathematical modelling with applications in diseases treatment
• Application of fractional differential equations for analysing disease problems
• Implementations of ordinary, partial, and integro- differential equations with optimization methods for disease problems