Some people are asking if it is permissible for men to not attend the masjid for Jamā‘ah or Jumu‘ah on account of fear of being infected by the coronavirus or transmitting it to someone else.
“Sickness” is a valid excuse to not attend Jamā‘ah/Jumu‘ah. Sickness in this context refers only to such sicknesses that make it intensely difficult to attend the masjid. Hence, ‘Allāmah Ẓafar Aḥmad al-‘Uthmānī explains the sickness that excuses one from attending Jamā‘ah as follows:
والمراد بالمرض ما يتعذر به الحضور إلى الجماعة
“What is meant by sickness is that which makes it intensely difficult to attend the Jamā‘ah.” (I‘lā’ Sunan, Idārat al-Qur’ān, 4:204)
Similarly, ‘Allāmah Ibn ‘Ābidīn describes sickness as falling under such excuses that make it near-impossible to attend, as opposed to excuses that do not. (Radd al-Muḥtār, Dār al-Thaqāfah, 3:511)
Likewise, an old man who finds it difficult to walk and is thus not able to attend the Jamā‘ah without intense difficulty is excused from attending Jamā‘ah and Jumu‘ah. (Ḥāshiyat al-Ṭaḥṭāwī ‘alā Marāqī al-Falāḥ, Dār al-Kutub al-‘Ilmiyyah, p298; Radd al-Muḥtār, 3:514; Ḥalbat al-Mujallī, 2:513-4)
If someone has a minor cough or minor fever, or has no symptoms at all but is merely concerned he may be infected by, or transmit a virus to, someone else, this is not a valid excuse for missing Jamā‘ah/Jumu‘ah. (Radd al-Muḥtār, 9:587) Yes, if the circumstances are such that the people at the masjid will very likely be offended and take issue with one coughing/showing signs of being infected by a virus, then it will be permissible to not attend. (Radd al-Muḥtār, 4:216-7)
The reality is it is always possible to be infected by or transmit an illness when in close proximity with others, hence the argument that this would open the door to fatalities is merely an emotional argument as this is always a possibility. If the chances are increased somewhat, this does not change the ruling of Sharī‘ah. Yes, one can take more precautions within permissible means, but it is not a valid reason to dispense with something that is a known obligation.
Even if contracting the virus was valid grounds to not attend Jamā‘ah or Jumu‘ah, it would only be when one is almost certain or very sure that this would happen, and almost certain or very sure that contracting it will be debilitating or fatal, and not just a minor illness. (Radd al-Muḥtār, 9:588) In this case, the probability that an individual will contract the virus by attending the masjid is low and, even if contracted, the probability that it will be debilitating or fatal is also low (less than 50%). 
The author of al-Hidāyah says:
لا يجوز تأخير الواجب لأمر موهوم
“It is not permissible to postpone an obligation for something speculative.” (al-Hidāyah, Idārat al-Qur’ān, 4:331)
‘Allāmah Ibn Nujaym explains, with reference to the rulings of Fiqh, that the feeling or judgement that is given consideration in Sharī‘ah is a dominant judgement or feeling (ghalabat al-ẓann), not merely any judgement (ẓann), let alone a mere speculation (wahm). (‘Umdat Dhawi ‘l-Baṣā’ir, al-Irshād, 1:199-200)
The solution to the current situation is not less dīn but more. It is a very telling sign of our failings as an ummah that people are fleeing from the masjids, the most beloved of places to Allāh, but rushing to the markets, the most detested of places to Allāh. The Prophet (ṣallallāhu ‘alayhi wasallam) foretold that our downfall as an ummah will be on account of “love for [worldly] life and hatred of death.” (Musnad Aḥmad, Mu’assasat al-Risālah, 37:82)
A Muslim should have full and complete conviction that no illness or calamity will befall him if Allāh does not wish it. And by turning to Him and gaining closeness to Him, Allāh will avert calamities and trials and, even if Allāh decrees that he suffers a trial, Allāh will make it a cause of blessing and mercy, and make it bearable and even pleasurable.
Allāh says: “Whoever adopts taqwā of Allāh, He will make a way out for him.” (Qur’ān, 65:2) It is reported that the Prophet (ṣallallāhu ‘alayhi wasallam) said: “Be mindful of [the commands of] Allāh, Allāh will be mindful of you. Be mindful of [the commands of] Allāh, you will find Him before you. When you ask, ask Allāh, and when you seek help, seek help of Allāh…Become acquainted with Allāh in ease, He will recognise you in difficulty. Realise that what missed you was never to befall you and what befell you was never to miss you. Realise that [divine] aid comes with patience and relief comes with distress and that with hardship comes ease.” (al-Arba‘ūn al-Nawawiyyah, no. 19) It has thus been reported from ‘Alī (raḍiyAllāhu ‘anhu) that: “A slave should have hope in nothing but his Master and fear nothing but his sins.” (Muṣannaf Ibn Abī Shaybah, Dār al-Qiblah, 19:157)
This is not a time for panic, but a time for introspection and rebuilding our broken connection with Allāh and preparing for the ākhirah, by making tawbah & turning away from all sin, observing taqwā & increasing our efforts in obedience to Allāh, giving in ṣadaqah, observing ṣabr, being punctual on obligatory and optional ṣalāh and increasing in tilāwah, dhikr and optional worship. May Allāh give us the correct understanding and enable us to follow the correct path and protect us from incorrect understandings and enable us to stay away from incorrect paths, āmīn. It should also be noted, there is no certainty that holding congregations will cause the virus to spread. Such things are entirely in the hands of Allāh. It may be because of maintaining congregational ṣalāh that Allāh halts or reduces the spread of the virus.
Some people have proposed that the fear of being infected by the coronavirus is comparable to the excuse of heavy rain, which the Fuqahā’ have listed amongst the excuses that permit one to not attend Jamā‘ah/Jumu‘ah.
Heavy rain is a real and obvious inconvenience for a person making their way to the masjid, and may even act as a barrier. (Radd al-Muḥtār, 3:514-5) Fear of being infected by the coronavirus is in no way comparable. It does not cause any real and obvious inconvenience (only an imagined and speculative one) and in no way functions as a barrier to making one’s way to the masjid.